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Steel pipe runs above the head, under the plastic cover on the return from the rad. Probably a mix of corrosion and some sort of rad sealant, certainly nothing getting past it.

Finally sorted.
The return metal pipe that runs across the top of the head was completely blocked, I tried to unblock with acid, coat hanger but well and truly blocked. In the end used some 10mm copper tube in its place. Not pretty but does the job.
Once again thanks for your help.

Thanks again for the replies. Spent a few hours on the car today. Tried new thermo switch.... Still no fans coming on. Checked both switches by applying heat to switch whilst plugged into fan and fans work, therefore must be a blockage of some sort so the thermo switch doesn't get hot coolant on it....

Thanks for the reply.
I'll get another thermal switch and try again.

Thanks for the reply. Car has single fan with 2 speeds. I've replaced thermo fan switch, but fan still not working as it should, ie. low speed for a/c and temp., and high speed for increased temperature. I have disconnected new thermo. fan switch, and bridged plug and high speed and low speed work. ...

OK, spent a couple of hours on the car. Flushed system through - plenty of particles came out. Bled system again, run for 10 mins at tick over with expansion tank lid off. All good, temp. up to middle, all hoses hot, heater working fine. Then increased revs and temp goes up, heater goes cold and coo...

So, finally got time to look at the car. Put power directly to fan- fan works. Bridged fan switch plug- fan works. Therefore, appears I need a new thermostatic fan switch. Bought that, fitted all okay, no leaks. Tried bleeding the system, squeezed hoses, removed/refitted coolant sensor a couple of t...

OK, thanks for the reply.
I'll look into the fan not working and replacing o ring on the sensor. Probably won't be for a few days because of work. Thanks again

OK, thanks for the replies, sorry for the delay in replying, I've been busy with work. So, managed a couple of hours on the car today. The fan temp. sensor /rad join are stained so there is a leak there at least. Bumper and cross member off, trying to get the fan switch plug off to check fan, but di...

Thanks for the reply. I don't know when the belt+pump was replaced. Just picked up new expansion tank, if weather permits, I'll make a start this afternoon and try bleeding system through. Fan is free moving, so would indicate problem with power to..... I've taken on board your suggestions, will plo...

A family members car is overheating, so I said I'd have a look. Took it for a drive, temp. went up to red/warning light. Fan didn't kick in? Stopped car, loosing coolant from under expansion tank, (hoses+clips look OK, so assume its tank that needs replacing). I'll change expansion tank tomorrow, I'...