Wow, thanks for all the replies.
Naively, i had thought all i needed to do was remove the crank from the chainring using the 4 fixings and fit a new one, if i could find one!
I cant remember when i last had parts on the drive train changed. It might be prudent to change the chainrings, cassette, jockey wheels and chain when i sort this out, so everything is back to a known good condition.
Id also thought about changing to a SRAM GX Eagle groupset with a new BB, which was where my original comment about costly came from.
@montgomery: thanks for the link but i note there isnt a 175mm available at the moment. Would there be any noticible difference between 175mm and 170mm? Id need a new BB and also a crank for the other side?
@Endomick: I am in the UK. Is this just the drive side crank? My cranks are 175mm, is yours the same?