Thanks chaps, pulled my finger out and did some research myself today. I'm currently running 225/40/18 which I believe is what it came with (although didn't buy it new, so they could have been changed I guess). Some good info here - http://www.wheel-size.com/size/skoda/superb/2013/#trim-20tdi-eudm
According to wheel-size.com my (2.0 TDi 170) 18"s are 7.5j ET 46 and the superb 17"s are 7.5/49, whereas these are 7.0j ET 54, so the result would be 1.6mm further in than my 18's (according to willtheyfit.com), so pretty much nothing, however I think I'll wait for a 7.5j ET 49 set to come up so that they are OEM fit and I don't feel the need to tell my insurer...whether I should or not is a different matter!!!
Thanks again for all the info.
John