The rcd on my consumer unit has started to trip on loads over 1kw ,its just the upstairs sockets which used to be fine with three heaters.I can't find water or obvious damage.its currently a 30mA,can I change it for 100mA?

A 100 Ma rating of a RCD is only used for systems that implement the TT Earthing System, and lower rating RCD's will also be needed for those sockets considered as high risk.

A qualified electrician should be called in to test your ring main to determine what may be the cause of the tripping, and measure the fault current flow to size the RCD properly. Anything beyond this is only guessing, as each system is unique.