Thanks a lot everyone. I have to confess that I rushed out last night straight after work and bought myself a saddle - a Specialized Expedition Gel saddle from the Bike Co-op. Once I'd got it into the right position (as far back as possible and tipped down to the rear so I don't slide forwards) it was very comfortable.
I chose the Specialized because with the old saddle I found that, comfortable as it was, I couldn't go for more than half a dozen miles without starting to get all numb and tingly in the unmentionables, which can be very off-putting. The Specialized is one of those saddles with a groove down the middle - should solve that problem. Time (and distance) will tell me how well that works. I see that Selle Royal does similar saddles, and Gazelle saddles are also grooved to a lesser degree.
Re Brooks saddles - when I bought the bike it was offered with a Brooks saddle and I turned that down in favour of a standard Dutch saddle (quite possibly a Selle Royal). I know Brooks saddles come highly recommended but I wanted something I could leave out in the rain on a busy street without it getting either ruined or nicked.
Ed1 - that's a seriously impressive discount.
Thanks everyone for your help and offers of help.