Buzzer of Doom?

[t25t25t25]

Hello, my T25 1990 DG petrol manual, needs help. Turning the key to starter brings on only a solid red oil light and the buzzer of doom!
The starter motor does not run.
There is a quiet click from no' 42 relay ( coolant? ) when ign is turned on.
Battery is good and fully charged.
Other electricals are ok - bright headlights, wipers and heater motor ok.
Update today - now no lights or buzzer but the brake red bar warning light is on all the time - ie without the key in.
What does relay No. 18 do? It energizes when the key is turned to starter position and is a 40 rating.
There is no 12v at the starter solenoid red/black wire.
Does not seem to be any 12v at the engine ie coil etc.
I may have found it! Engine earthing very poor. Hope this is the issue.
any ideas very welcome.

[Mocki]

Is this a sudden occurrence?
Was it ok one day and then not ?
After you have checked, cleaned and established good earth points all round , next place to look is positive battery terminal , and then ignition switch .