Thanks for the fast reply's.

It gets pretty hilly around here at times. So I'm thinking changing the crank set is a good idea. And while I'm at that, I will change the Ball Bearings, another 'first' for me.
But first I will sort the cassette, and fingers crossed my rear derailleur will work fine (although I'll do some Internet research first - see what I can find)

OK with a little searching I found this:
http://www.sprockets.uk.com/deraille...ed-p-9323.html
I believe that would be the same as mine, and I think it is saying the maximum it can deal with is a 28T and minimum is 11T, but I am not sure what it means with the front chain-ring difference of 20T?