Did you keep the old valves? You could try fitting one to the spare, which the car should then recognise and be happy. Cheaper than a trip to the dealer. Ex AA Series III LWB Safari - Gone
300TDi Disco (bought new - terrible car) sent back after 18 months
Freelander 1 Estate - leased, given back at end of lease
200TDi Disco (bought from a mate with 100,000 on the clock) - Gone
Disco 2 TD5 - sold and exported to France
FR2 TD4 GS - Gone
FR2 SD4 HSE - Now changed for a DS
New model ex-demo Evoque S 180 in white
Unable to order a new DS, so gave up. Now have a Volvo S90 Recharge.

Brilliant minds think alike. I actually attempted this last week. Shop was able to mount the tpms in the spare. Their machine can "see it", and in the car it's even displayed as a sensor, but the shop in unable to "assign" it to the spare tire for some reason. Still have the derror message on dash unfortunately.