Is it to late for re-potting bamboo

If its bending the pot, it'll only get worse, so you'll have to do something about it now. Get it out and put in something bigger, or chop a lump off and repot.

Would John Innes 3 be ok, or would it be better in m/purpose?

John Innes 3 will be fine (as long as its not been hanging around for ages) - but if you've got some multi purpose, I'd mix the two together to lighten it up a bit.

Thanks Bamboo!

;-))