Quick update to let you all know how awesome your help has been - I put it on the big cog at the front and all is well and also on reflection, given my ride length is so short these days (I have a 3 year old son) then making the ride harder going is a good thing really . My LBS will be getting a visit so I can get him to teach me about the points raised in the replies about chains and cleaning (he's an old school enthusiast).

Thanks everyone