Various plans on T-Mobile too
PAYG Free BlackBerry Sim Cards on T-Mobile - Get a free BlackBerry SIM card on Pay As You Go
30 day SIM-only plans require an extra £5 a month for the BB service, starting at a base cost of £10 a month, depending on how many minutes/txts you want too.
Beware when buying a BB from Ebay, that you might have to get it deregistered from the carrier it may already be locked to. This is in addition to getting the phone "unlocked" so it can use the SIM of your choice. If you have the option, buy a used phone already on the carrier you plan to use, or a phone known to have come not from a carrier (like Carphone Warehouse)
And Orange and T-Mobile users can now roam on to the other's network without extra charges now (for GPRS, not 3G), by the way.