Disco 1 My (very early) 300v8 won't start

I'll start by saying I think I know what's wrong, but....

I went to get in my disco one morning and the plipper wouldnt work, but as it happens, she was unlocked so I managed to get in her.

It is the older single button system wiithout a spider

I turned the key, the dash lit up, but all I got was a couple of relays clicking, but the starter wouldn't turn.
I assumed that it was the batteries in the plipper, I changed them but no luck.
I also noticed that the LED under the radio doesn't light up ( but it might not have for years, i'm not sure)
The electric windows don't work,
The heater doesn't work,
the central locking doesn't work.
The radio doesn't work.

The engine can be started by applying 12v direct to the solenoid terminal on the starter.

I know I could get it to start by running a new wire to the starter and bypassing everything else, but I don't want to bodge it and I want to get everything like electric windows etc working...

(The fuel pump relay isn't standard so the immobiliser may not have an effect on this any more)

Does anyone have any ideas?

Horn and head lamps work still? but we will run through the usual suspects, for the engine not starting, the vehicle is still immobilised, as cranking is inhibited, or a flat vehicle battery, plus iffy battery, earths, iffy fuses/ fusible links and wiring harnes connections for the electrical stuff that doesn't work.

Just a little bit to check out

It's not a flat battery - i've got twin batteries and a T-MAX link which shows they're charged. Also you can start it by bridging the starter solenoid.
Everything died at once so i don't think it's individual connections to circuits.

Check the power feed to the alarm, mine totally immobilised itself this week and transpired to be a corroded connection causing no power to he alarm module. This was on a newer one though (2 button fob) but the alarm module was under the glovebox to the left hand side and the offending connection was behind the kick panel next to that. Hope this helps

Just realised i never posted the SORTED! bit....
Trns out the fusebox had corroded, tapped it and nearly every wire fell out the bottom :S put a new fuse box in ( well my friend did as I was having ti deal wit paying customers ) and it worked fine...