Many thanks for your replies.
I don't want to spend loads of money on a road bike, and then even more on something to make it not go anywhere, so although I can see that it suits some, it wouldn't be what I am looking for.
From what has been said here, and also on another forum that I post on, I think I will stick with what I have, as it seems to be doing the job OK and takes about 20 mins to get uncomfortable rather than the 5 mins that most seem to find on exercise bikes :?
Very grateful for the comments and advice, and I hope others have found it useful too.