FYI If you don't have any luck getting hold of a used one, we did some recent looking around and found the following cheapish new options (that take a 10-speed cassette):
Decathlon do a rear wheel, but not sure whether it can take a 10-speed cassette
http://www.decathlon.co.uk/rear-700-wheel-cassette-id_8005016.htmlRutland cycles on ebay do a rear wheel:
http://www.ebay.co.uk/itm/SHIMANO-TIAGRA-10-SPEED-ROAD-RACING-BIKE-REAR-WHEEL-700C-MAVIC-REAR-WHEEL-/380448027835?pt=UK_sportsleisure_cycling_bikeparts_SR&hash=item58947834bbOr Ribble do a cheap front and rear for not much more
http://www.ribblecycles.co.uk/sp/road-track-bike/Shimano-R501-Clincher-Wheels-Pair/SHIMWHFR202