I changed my 93 XJ pump last year so can give you a few tips. This job is much easier with the tank removed, you will need to reduce the amount of fuel as low as possible before attempting to remove the tank. It can be done with the tank in place but it makes the job much more awkward. Lighthouse 01953 681237 should have a suitable pump. I got mine from the USA, not sure it was worth it after adding all the taxes.
The pump is held in the tank by a locking ring, you will need to use a brass punch (sparks and petrol are not a good idea) to tap it anti-clockwise. Once the ring is free, the pump, bracket, fuel level float can all be removed with care and a new pump fitted.
Cost? I would think around £100 ish. Much more info can be found at
http://www.naxja.org/