Btwin MTB bottom bracket . . does anyone know off hand which size fits ? Its square tapered are, these all fairly standard ?
No - Square taper bottom brackets are not "standard" as far as shell width and spindle length are concerned. MTBs normally need a 73mm, whereas the link offered to you is for a 68mm (surely
Decathlon is a more reliable source than asking here). And you will struggle to find out what the fitted spindle length is without (see below).
But if you're asking this question, you're implying you'll be replacing it yourself, in which case you'll already have the tools. Pull out the current one and it'll have its spec printed on it. Otherwise your LBS will sort this for you. BBs can be a challenge to get out, especially if the left hand side is plastic. I wouldn't purchase a BB (size/spec) based on the advice of randoms on a forum. The Shimano UN55 bottom bracket is often recommended (and the couple I've had in my current road bike have given me 15,000+km, each).