bath overflow

That's a popup waste ,probably operated by a cable connecting the cap you speak of to the waste down below . You may well find that the cap is held on by a small screw which you will see if you lie down in the bath and look underneath the cap .Failing that you might just need to use brute force and ignorance and prise it off . Anyway I would suggest you get rid of it and fit a normal waste with a plug either on a chain or not as you prefer .

Incidentally why do you need to remove it to change a tap ?

Stuart

thanks stuart i will try that i am trying to fit a new shower mixer and dont think i can undo the nuts on the tap because of the tight space