I've now completely stripped the original box and cleaned loads of disgusting black sludge from the internals of the clutches, valve unit and servos. All the friction materials were in good condition with manufacturers lettering still visible, so they were washed and reassembled. The servos are easy to clean as they are just pistons in a housing.
Don't be freaked by stripping the valve unit, no matter what the Haynes manual says. Get loads of newspaper to put on the bench, kitchen towel to mop up the gooey oil, plastic sandwich bags to put the bits in and a marker pen to label the bags. Also something to wash the bits in (an oven roasting tray or washing up bowl) and some paraffin/degreaser/white spirit. An air line is useful but not essential.
Follow the instructions in the Rover manual exactly (ask/email if you want a copy) and bag and label everything as you remove it. Be meticulous. If unsure make notes/drawings, but you'll probably find bits will only fit in the right position.
Wash everything individually to clean out all the crud (use airline if available), and find somewhere clean to reassemble it. Again, follow the manual to the letter. It's not difficult, but you have to be methodical.
The whole job took about two hours and I managed to find a potential cause for the dodgy 1-2 gear shift evident for the 7 years I've had the car; the "1-2 shift piston" was sticking (sorted with fine emery paper).
A set of gaskets from David Manners for £20 (Jaguar parts are cheaper!!) and hopefully once I find a way of getting the car in the garage, I can put the old (but refurbished) box back in and get it back on the road for a lot less than the £475 quoted earlier in the thread.
Fingers crossed,
Mike