I think you're best bet is to find a good, local, shop and go and see what they recommend, sell and support.

If you're new to all this having a good relationship with a local retailer will probably save you both money and heartache..

What's a decent price? Can change depending on if you're a millionaire or a shop assistant..

Have a look at the boardman range at halfords though. Good value starter bikes.

do you have a budget in mind? you thinking Hardtail (front suspension only) or Full suspension?

you will get more for your money if you stick with the Hardtail setup and it will make you a better rider in the long run than going for a full sus IMO

a good baseline would be about £500 for a good Hard tail with 80-100 mm of travel. you can get a bargain on here if you don't mind going for a second hand bike.

I would think that [url= http://www.decathlon.co.uk/EN/sport-mtb-3962638/ ]the Rockrider from Decathlon[/url] would be a good staring point, again, what is the budget?

What's your price range?

Carrera Vulcan V-brake from Halfrauds would be the cheapest MTB I'd naormally buy/recommend for off-road use, £270 at the moment - but that decathlon for £200 looks a bargain.
Both would be a good place to start and upgrade from, as you need to.

Best recommendation at this point would be make sure you get a frame that fits you well, nothing worse than starting on one that too big (like I did).

Go to a local bike shop, and test some out. Decide how much you want to spend and go from there. Any entry-level bike from a big name is going to be good.

purely my opinion here but the suggestions above, whilst probably good value bikes, are not sold by what I would call an LBS (local bike shop) but large faceless chains where you are unlikely to get the same level of after care that you should get from a small independent retailer. Go into a few near where you live and chat to the staff. See how you get on and whether you get a good feeling in any particular store and the bikes they stock. Tell them about the type of riding you want to get into but be realistic that this will be your first bike. I would also go for one of the larger brands first time out (Specialized, Giant etc) because they offer a good balance of decent spec at a decent price.

Edit: So what he said ^^^

I've just got my son one of these

http://www.evanscycles.com/products/specialized/hardrock-comp-disc-2010-mountain-bike-ec019398?query=hardrock

and I am well impressed for the money

Thanks for comments so far. Was thinking Specialied, round 400/500pound mark. Front suspension probably, and new.

Specialized make some great bikes, but you paying a preminum for the brand name. I still recommend the Carrera/Decathlon's at this price range due to the spec's: Carrera Fury & Decathlon 8.1

Try bikesandprams.co.uk

Specialized are very good entry level bikes.

Thanks Z1ppy, looking up the Decathalon. Looks good.

Having introduced a new rider to he hardrock, he's never looked back and has recently stuck some bigger forks on to have more fun