Hmm...thanks for all your input, looks like the overall consensus is that I could get away with just changing the chain....
Rang another bike shop up today, just for a 2nd opinion, who advised I might get away with it, but would then look at having to change the cassette a year or so down the line anyway, so, overall, they again recommended doing both.
Difference was, the price quoted was less than half of shop number 1, so I might go for it... My friend is taking her bike in tomorrow to get it sorted (same issues), so we see how she fares once they have seen the bikes in the flesh. I let her be the guinea pig - if I'm happy with what they do to hers, mine can follow next week!
One day, I'm going to buy a cottage in a small village and become its idiot!