HELP!! does 5mm really make that much a difference?

4 or 5 inches in leg length………..what you looking at buying?

5mm lower/higher seat – does that make a difference?
5mm nearer/away pedals from the ground – does that make a difference?

http://www.machinehead-software.co.uk/bike/cranks/cyclist_crank_length_calculator.html

Tho' not sure if road and mtb ought to be the same/different?

Simple answer is that it makes very little difference.
using that calculator thingy a:
31" inside leg should use 163mm cranks
33" 170mm cranks
35" 176mm cranks

I am 5'11" and should use 165mm cranks according to that. I use 165mm of my DH bike and 170mm on my 4x bike. Basically long crank arms give more leverage but less groudn clearance. You can then adjust seat height and also the thickness of your pedals.

But basically it makes not a jot of difference really. If your short use short crnaks it your tail use short cranks for DH but long cranks for XC.

Dirtynap cheers buying 170mm

I' ve never understood the convention that road bike "standard" was 170 but mtb was 175. the standard explanation is that the longer cranks give more leverage for climbs, as if dropping a gear wasn't an easier option.

I' ve never understood the convention that road bike "standard" was 170 but mtb was 175. the standard explanation is that the longer cranks give more leverage for climbs, as if dropping a gear wasn't an easier option.

That's part of it. Convention is that shorter cranks make it easier to spin which is what you do on a road/track bike. Longer cranks give more leverage hence why they became standard on MTBs.
Of course all this was back in the day when you had 5 or 6 gears (7 if you were minted), nowadays road bikes can have 33 if you have 11sp Campag with a gay triple, MTBs are at the 30sp stage (just) so it's less of an issue.

Shorter cranks help for DH cos they give fractionally more ground clearance, on track bikes 165mm is normal, partly for the spinning reason, partly for clearance (clouting a pedal as you go up the banking is not fun!)

175mm for all my bikes except the track bike and the roadie SS where I have 170mm.

fair enough on the pedal clearance issue – but I don't think I've ever seen anyone acknowledge that those "extra leverage" mtb cranks are going to be harder to spin and that your top gear is going to be slower than it would be with shorter cranks – so I still think the real issue is getting your gearing range right.

as for back in the day, surely even the first stumpjumpers came with triple chainsets?

and your own bike set-up? aren't you missing the "extra leverage" on the one bike where you could really use it? I'm guessing most singlespeeds (assuming it's not fixed) are more compromised at the low end of gearing than the high end. not saying you're wrong at all – if anything, setting up a SS like that just makes the point that it's more a gearing issue than anything else