If it's anything like a MK2, the 2nd fuel pump is under the car just infront of the drivers side rear wheel could be that..? Really the only thing to do though is jack it up and trace the leak back. Fuel tanks are usually easy enough to remove should you need to.
I did art and media, I pissed around in both and got a pair of E's. Have my GCSE results changed anything in my adult life? No! Just lie and say you got all A-C's* it's easy, nobody has ever checked, ever (including college)! *Don't lie, and try very hard at school.
Top the coolant right up, then run your engine up to full temperature with the expansion tank cap off, squeeze the radiator hoses while it's warming up taking note the temperature of the top and bottom hoses - the top hose should stay cooler than the bottom until it starts to get up to temperature. Once it gets hot enough the coolant will boil over unless your radiator fan is mighty efficient! At this point switch off and top up the coolant as it goes down. If you've got to this stage and the top radiator hose is still cooler than the bottom one, your thermostat is indeed faulty. See if that sorts anything for now though... Then - I'm not sure if 6n's had heater bypass valves, if you look at the 2 pipes that go through the bulkhead are there big plastic bits inline with a little pipe going between them?
How long is a piece of string? And make sure it's started from cold and listen for any nasty knocks, and be checking round the sills, arches, boot floor, and petrol cap for rust 'rocco's looking good Robin, still needs a considerable drop though