[CNHSS1]

the temp thing could be the guage mismatched to the sendor. The nissan senders make the guage read very cool, normal is just below halfway generally, although they all vary somewhat, Good ol Reliant... Think you need a metro sendor to suit the dash.
If its oil smoke check that the turbo castings arent caked in oil cos even if you cure the leak, itll still smoke but you wont know it. Remove the two heat shields (main one and smaller one underneath/near chassis rail) and have a good look from underneath. A good steam cleam may well make the daignosis a hell of a lot easier.
Its a Ford header tank, so probably very similar to those used on your Quantum.
Difficult to tell if the heaters are working anyway! they are poor. If cars used infrequently as most scims are, then the heater matrix will be full of shit. Put some of that rad clean in the secondary filler cap (nearest thermostat) and use for a few days, then flush the whole system throughly with the garden hose and then fluch the heater matrix again separately with the garden hose connected first to the inlet stub ( pokes thru the bulkhead next to heater blower under bonnet), then repeat on the exit stub and reverse flush it. Makes a big difference. Stops hotspots too. Refill with good antifreeze (50/50 water a/freeze) suitable for an alloy head.
thermostats rarely stick (its the nissan bits that are reliable!) but they are only a few quid anyway.