VmanC thanks that's really kind of you to offer! A friend who used to have older cars gave me the number for a guy a few streets away so I took it to him this morning for a third opinion. I went armed with all the advice from everyone here and told him I would be running any quote he gave past some experts
He seemed quite genuine and spent a long time taking me through everything and showing me the engine. He then showed me the 'plugs' (which I assume is what the mechanic last week said needed replacing) and they are very rusty! He has quoted £70 to replace all 4 and clean the coolant tank out. I mentioned I have a spare new water pump pipe in the boot so he said he will fit that at the same time as he will need to take the old one off to access the plugs properly. I asked about the stalling problem again and he said he will clean the carburetor to see if it helps but that it is most likely a seperate issue.
So basically will do these 3 things for £85. Not too bad I'm thinking? Better than £150 the last guy quoted to replace one plug or the £350 my MOT guy quoted for head gasket which nobody else seems to think needs doing.
Hopefully this will rectify the leak and I will have found someone I can trust! Fingers crossed. Thanks again to everyone, your advice helped me make an informed decision and has (hopefully) prevented me from being ripped off. He will carry out the work in the next week or so at which point I will report back the outcome!