From what I have read here over a period of time if you have a good credit rating and treating this in isolation it makes little difference as far as helping your credit rating.
I take a different approach and say do I 'need it' and if not why not reduce it for 'fraud' + reasons - £750 less at risk if you reduce to £250? That's the reason I reduced the overall amount at risk on my credit cards. If I want to increase for any reason I would just ask and be surprised if there was a problem. In any event the increase would be to get the safety of the CC so if necessary I could pay money into it.