Back again! M+S flashing

Hello everyone. Feel an arse only coming here for help, but I know 0 about Land Rovers....

The old dear's still awaiting an ignition barrel, as old one kept jamming, so got a bent bit of metal to start it with at the moment

Earlier today M+S started flashing and it didn't pull.
Checked handbook and said if it persists get it checked urgently - a gearbox electrical fault. I took it for a spin, and it was fine.
Anyone had this, or know what the favourite culprit is?
Does it need plugging into diagnostics ideally?
She had to borrow my M3, hopefully she didn't try off-roading with low profile tyres lol!

Any help is as always greatly appreciated, and the thing keep going wrong really gets her going off on one

It needs to go to a dealer or someone with T4 to read the fault codes

lack of power is because it defaulted to 3rd gear, a type of limp home mode

normal causes for this are CAN time-out, XYZ switch fault or output shaft speed sensor failure.

If when the lights come on, turn the engine off and restart and then its ok then its probably a CAN time-out fault which baiscally means, the gearbox ecu was expecting information from the engine ecu but didn't get it in time, there is updated software for the engine ecu for this (reprogramming it)

Top info
Had an auto that kept going into limp mode, once at bottom of multi story car park ramp, that was fun

Should have said, it's a W plate TD5. Best get it booked in then, could be done whilst new steering barrel is done!

Cheers.