Got it sorted!

I managed to source a replacement inlet manifold from the Guru. I bought some more Hylomar, an thermostat and some Radweld (the radiator I'd fitted was leaking slightly) and set to work. Things went quite well until I came to fit the thermostat. There was a rubber seaing ring on the thermostatat and when I bolted the housing down it cracked! Fortunately, I had a spare so that was put into service. This time I removed the sealing ring and used Hylomar instead. To avoid the problem which I'd encountered in the first place I used copper grease on the thermostat housing bolts when I tightened them. The troubles didn't finish there. The top hose was leaking so I ended up replacing that with a spare one I had. After that I refilled the cooling system and the car has been fine since. Better still, it is running cooler than before. The needle sits above the notch on the gauge only rising to just above halfway when idling in traffic. I believe that replacing the thermostat has made a difference.