I've had a couple of rattles on my Mk1, so not sure what is common between the mk1 and mk2's.
1st has been the back plates on the brakes. They are held onto the hub by 4 or 5 little bolts. One of my backing plates rusted away round a few of those bolts and they started to rattle. Pulled the plate off in the end and replaced it with one of some suspension spares I already had.
The other thing was on the exhaust, just before the back box area. There is a weird metal and rubber weight bolted to the exhaust on each side. It is apparently something to do with being a resonance damper. Anyway, the washer had perished and had dissappeared so was quite loose. The only way I could get it to rattle when ithe car was in the air to find the rattle was to bang the cat quite hard with my hand at the front end of the exhaust system. Angle grinded the old bolt off and then put a new one through and tightened up, the rattle had then gone. As a quick check, you should i think be able to reach these weights without jacking the car, they will be roughly in line with the back axle on the exhaust system. Give them a wiggle, if they wiggle, they are loose.