Tamper problem with Optima alarm?

I have a very old Optima alarm (was in house when we bought it 6 years ago). 4 zone I think. One evening, the control unit (where we input the PIN number) started making a rapid beeping noise. The tamper light was on and the LED display had a 6 in it (or a 'b') - I am guessing this means 'Bell'. Putting in our PIN number stopped the beeping, but didn't clear the tamper fault. After about 15 minutes the whole LED display and lights went out. All the PIRs are dead (no LEDs flashing when I walk past). The LEDs in the bell box outside are also not working.

I have since found the master control unit (where all the wires meet) in our airing cupboard. Had a peek inside, checked fuses, nothing obvious. I'm no electrician, so I don't know what to look for (though I do have a multi-meter if that helps with testing any diagnoses). Most of the circuit board is obscured and I can't see if any components have blown. No obvious loose wires. Put panel back on master control unit and turned power back on. Our little unit downstairs (where we enter the PIN number) sprang back into life for about 30 seconds. I entered PIN number, but I got the tamper light and a 'u' code in the LED display. Then the whole thing went dead again.

Any ideas folks ? I am guessing the back-up battery in the master control unit was the reason it lasted for about 15 minutes (and then the extra 30 seconds), but that it is now flat.

I am wondering if the transformer has gone (so no power is getting to anything). Might that be right ? How can I test it (preferably without the mains switched on) ? Does the 'u' code mean I haven't put the panel back correctly on the master control unit ?

Any advice really welcomed. Thanks. Andy