Old frame has a 72mm BB, new one is 68mm. I have an old Shimano M440 Cranset fitted to the old frame – can I change them across?

If not I presume I would need a new BB so would I just be better off getting a new Deore crankset from CRC for £50?

Going on a Maxlight XC frame.

You'll need a new bottom bracket to fit the 68mm shell but with the same axle length as the current one (probably 113mm or 118mm). You should be able to pick up a new BB reasonably cheaply (I assume it's a square taper) but a new Deore HTII crankset will include a HTII BB as well as the crankset.

You can change them but why? you'll need new bb anyway. And how are your rings!? If they're Knackered you'll need new casette & chain probably too!!

Deore Crankset comes with BB I think "dusty trails".

Anyone know if there will be a significant weight difference between my olf M440 and a new deore crankset.

How do I know what axle length it is?

Take out the BB and measure it- it'll be either 113mm or 118mm more than likely.

A new BB is £25 RRP (not sure who's doing it for what), and you may as well go for a new chainset as you'll get new rings and have more of a new bike feel.