Hi,
There are more than a couple of ways to go, but two i can think of are:
1) get a recon kit from ashcroft trasnmissions and do it yourself. Usually the kit contains all the seals/o-rings/gaskets etc needed. but you would also have to get the clutch plates for the innards as well. ~£50 to £300 depending on how much you need to get. Not a simple job, but possible if you take your time.
2) i think there is a ZF autobox specialist in tollcross/glasgow who do a 3 day turnaround on autoboxes ~£1000 ish.
http://www.mackie-transmission.com/commercialthere may be more out there.
the autobox on the td5 disco is probably the ZF hp24 which had electronic solenoid activation of the hydraulic "brain", might be worthwhile seeing if the electronics are damaged. Things like connectors etc. The controller is under the passenger seat and the connection is quite large and might have let water into the control unit.
cheers
Stephen