I think it may be possessed........

We have a 1993 Saturn SL which is kind of beat up, but still runs ok.

Used it in the morning with no problems at all. Came out after lunch and started as normal but the automatic seat belt on the drivers side got half way to the back position then stopped and went to home position. Then after a while did the same again. When it does this all the gauges drop to zero and the internal light goes on and off.

Everything still runs ok. With the ignition off the red warning lights all come on dimly. If I switch off the internal light the seatbelts go to the parked position and none of the gauges work.

I'm not hugely mechanically minded but want to try to get it sorted without taking it somewhere. We're gonna replace it in the next few months and I don't want to throw money at it at this stage.

Do we need a mechanic or a priest?

keep a eye on the battery light, sounds like a dead alternator or weak battery. also check ALL fuses.

I agree with sierrap-
get a multi-meter or a voltometer and check your battery both while
engine is cold ~ 12.5 DCV (if not -battery)
and while engine is idleing ~ 14DCV (if not -alternator)
and check all your fuses
it may be a short on the drivers side door ajar sensor, check your wires good.

yup.
alternator or battery
to keep the car running you need at least 10 volts
to start it you need 12.5
the autozone 1yr refurb is like 70 dollars and takes like 30 minutes to install. i dont care if they tell you it takes an hour i've done it 15 times myself... long story. but 30 min for a person who knows what they're doing.

Check the PAssive Restraint Module underneath the center console. Make sure it isn't cracked or has stains from a spilled drink on it.

Another possibility is a chaffed / pinched wire under the headliner that is shorting out,