Wonder if any of you car types can help?
Our trusted Ford Focus just started playing up in a very weird way - it seemed to hold the revs at the level they were last at, so if I pulled up to park, or changed down a gear, the engine would be revving at a much higher level - scary, and I guess a bit dangerous 
Drove the 5 mins home and am now clearly gonna get professional help but wondered if there was a common complaint that would cause a car to behave like this?
Thanks...
o2 sensor needs cleaning / replacing possibly.
Er, if it's drive by wire, faulty throttle position sensor? Otherwise, sticky/dirty throttle cable?
Thank you both. These things sound small, or am I being too optimistic? 
Does the O2 sensor get used for "choke" operation for cold starting or is there a temp sensor too? If so maybe this is faulty and the engine is running on choke all the time?
It will probably need some level of professional intervention to correct though.
Take out your drivers floormat and take it for a spin again or as above.
Mk1 focus? Mine hangs revs at about 1500rpm when clutch is depressed sometimes. Will either drop to idle on its own after a few seconds or just sorts itself out when you move off again. Had it checked, ecu reset, no joy. Doesn't seem to cause any problems though.
Phototim - that sounds like how it used to be, exactly the same. Start up the engine, weird lift in revs, then settled down.
Now it's keeping the revs up through gear changes and whilst stationary, but at more like 2k rpm or 3k rpm 
I had the same on my Volvo V40, it'd randomly hold rev's or even just rev itself upto 4k.
On reading and asking for advice it looked like the common problem was the idle control unit needed cleaning, was a pretty easy job even for a non mechanic like myself. But in the end it wasn't that as my engine management also came on and a sensor needed replacing.
As a guide, cleaning the idle control unit cost me £5 for the can of cleaner.
The sensor problem cost £90 at the local garage.
My first car did this (1977 Fiesta) - IIRC it was a sticky throttle cable.
Per Hora - try the floormat
Yes floormats can creep overtime in any direction.
Recently I experienced a toyota auris with its acceleration stuck wide open- clutch in, reached down-pull and freed the accelerator. OEM Toyota mats as well. Gobsmacked. If that hadn't worked I had my hands on the ignigtion keys as I didn't want a ruined engine!
Always start with the basics.
Pull the pedal up and see if that slows it down.
icv, or map sensor.
also check there are no splits in vac hoses etc too...
Air leak on the intake perhaps ? Breather pipes split ?
I had the same problem on my Escort. I didn't think much of it to start with but then one day, it got so bad that I managed to drive the 5 miles to work without using the accelerator as the revs were stuck at around 3k and the only way to get the level to drop was to turn the engine off 
I jacked the car up and emptied the oil out before removing the idle control valve, cleaning it with petrol & WD40 and then putting it back on and putting new oil in. That was in June and it's been fine since, although the engine does idle at about 1.5k when it's cold but it's an old car so it won't be perfect.
See Link on how to clean the idle control valve (note - link is for an Escort but the process is the same and the valve for the focus looks the same as the pictures)
It's not a big job and providing you've got a couple of axle stands you'll be able to get under the car to change the oil and remove the valve. It took me about an hour and half to do it and only cost £1 for petrol and £15 for new oil and a sump plug washer so it's worth a try before taking it to a garage and being charged an arm and a leg (and other assorted body parts).
OEM Toyota mats as well. Gobsmacked. If that hadn't worked I had my hands on the ignigtion keys as I didn't want a ruined engine
We had a recall on our Prius to fix the clips that hold the mat in place.
I had this on my Mk1 Focus, it was a common problem and wasn't anything to do with the floor mats 
My escort did this - a good service and clean the idle control valve sorted it ..
If my Landy did it I'd be worried - they like to run on their en engine oil
I had it happen on a 06 plate focus. They are fly by wire throttles and Ford said it was the throttle body that had gone.
Think the OP's is the mk1 though and yours is the mk2
I had this in my old Fiesta & as mentioned above a couple of times it turned out to be the idle control valve.
Took it off, cleaned it with GT85 and a load of kitchen roll and it was fine after that.
I wouldn't say I was a car expert but have mostly done my own mechanics
I've had this on numerous fuel injected fords including Ka's Mondeos, Granadas and a I think even a Puma, the idle control valve is usually the culprit. It will be a cylindrical unit held on with a couple of bolts and a multiplug on one end. Take it off and spray lots of carb cleaner into it and slosh it about. Shake out the excess and it should dry very quickly. Stick it back on and the car should start and run normally and the odd revving should go.
I wouldn't recommend using GT85 or WD40 as this is oil based and would probably allow more dirt to stick inside in the future. Carb cleaner is a quick dry spray that was recommended to me years ago.
If it's not that then it could be a temp sensor or something!
I used electrical contact cleaner (you can buy it from Halfords).
I used this on my old 1.9TDI when the revs were erratic.
Had the same problem on our Focus 1.8 Zetec
There was a split on a pipe that was allowing to much air to be drawn in when the throttle was off.
Before you spend any money check all the pipes sitting behind the carb.
What age & engine size is it? As usual there's some good & some bad advice up there ^. As mentioned, an air leak into the inlet manifold would give these symtoms. Also possibilities are vehicle speed sensor fault, clutch switch, power steering pressure switch - all of these could request a high rpm if faulty. Best option would be to get the fault codes read out, that should identify the cause.
All the above is wrong. Exocism will resolve the problem.
All of the above is wrong, it's the hand throttle. 
(Sorry, hora, unsettled spirits cannot possess a holy Ford, by Ford!)
Thank you all so much. Overwhelmingly helpful
Is it petrol or diesel? Assuming the floor mats are not holding the accel pedal down, and if there is a throttle cable, it is not sticky, then petrols will increase the revs with excess air, diesels with excess fuel. Both will have diagnostics available. Either way, as long as the speed increase is only in neutral, the amount of actual torque being produced will be quite small,
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