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If you're interested got new rad etc back on. Ended up cutting back some rust and repainting etc so took a bit longer than expected. Fitted new thermostat as well just in case. Temp from blowers is now much warmer, coolant returning from rad to expansion tank. Still overheating unfortunately but stops just under the 3/4 mark and returns to middle much quicker. Fan still not kicking in though which is a bugger. Got to test fan switch next. Mechanic replaced it but chance that's why fan not running.
Yes, some modern rads are shite. Check for fakes and unbranded never heard of rads. An example is getting motorbike, honda NC30 water and oil rads from china. Very hit and miss while car parts are being copied and are poor quality. There are lots of ways that rad makers are trying to prove that their products are genuine.
The cold internal heater rad does not inspire confidence too. Is it an air lock and is that air lock caused by a blocked rad? When you take the thermostat out back flush the system in the opposite direction to the normal water flow. Maybe disconnect the heater water pipes and back flush that to see if there is a blockage?
If the body is in good nick I'd try my best to keep it as decent MK2 golfs are only going to go up in value.