Thanks everyone. The exact problems are that the speedo is registering about 11 mph less than it should and the temp gauge is not moving. I've now tried Autotronics in Leicester and AC Tronics in Colchester. Unfortunately neither have my model on their systems anymore so don't have "looms and wiring" to properly diagnose the issues. Also, I've been advised by 3 separate parties now that it must be EXACTLY the same component serial number, so these other LS400s advertised won't necessarily be compatible.
When the problem with the speedo first manifested, the needle would not move until a sharp tap on the dash released it. But now, as I say, when it does move the calibration is out.
Don't know whether I should just put the cluster back in and keep an eye on the temperature and watch my speed like a hawk to avoid a ticket.
Mole