I ordered the Walker St Clair Cockpit Cover I'd seen on the Westfield World site: http://westfield-world.com/carcovers.html
It cost £54.95 and arrived in a couple of days.
The cover fits round the wing mirrors and windscreen wipers at front:
There is a bit of a gap where the sides are which I guess would allow rain to get in if there was a side wind. You can buy a couple of rubber suction hooks from some DIY place which you can use to hold in place but I don't think you need them unless it's going to be really windy:
and it tucks under the back:
It folds up into a nice small bag that I stuff behind the seat: