Thanks very much everyone for your really helpful replies. I'm tending very much towards a personal Crest account, as I do find certificates a bit of nuisance, particularly having to be at home to receive and respond to the paperwork at the critical time. And I'm reassured that Tom and Roger have used Crest without problems, though it would be good to hear from more Crest users.
I've just done some cost comparisons. Using Stocktrade, a £10,000 trade would cost £40 plus an annual £40 Crest fee, and Fastrade would cost £11.50 plus a £10.24 annual Crest fee. Using certificates, though, the costs are much higher: £110 with Hargreaves Lansdown and £175 with Barclays. With several trades of £10,000 a year, these differences are clearly very significant.
Tom's suggestion of a thorough comparison between Stocktrade and Fastrade and anyone else who offers personal Crest membership, is an excellent one. An analysis of how useful their websites are, and how easy they are to use, would be very helpful.