Many thanks for all the helpful replies, everyone. Sounds like I could have
fun experimenting with collected seeds, but that I run the risk of getting
something rather poor, especially if the original variety was an F1 hybrid.
I shall leave the experimentation until I'm retired and have more time for
it: at the moment I'm more interested in getting reliably good results.

So I shall certainly avoid seeds from any F1 hybrids, but perhaps will give
one or two other things a go.

Thanks again
Adam