Guys, maybe i misunderstood, but based on this statement

Today I Travelled 30 miles and the needle went into red about half a dozen times and came down again without me stopping.

The last couple of times just to try something I switched the ignition off and back on, whilst temp was in the red and whilst there was the drop in power caused by the issue, whilst I was driving. Immediatley i get the power back and the needle drops back slowly to where it should be rahther than it taking quite a bit longer without turning the engine off an on when i get the issue

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i understood that there are fast fluctuations between red and normal on the gauge...i'll stand corrected if i'm wrong but a HG failure would'nt cause overheating? ...and if the coolant/engine is hot it will not cool down faster by turning the ignition off/on ... the liquid/metal cools slowly regardless of the ignition ... or am i wrong

Sierra. The symptoms are correct but I don't really think it cools down as such. By turning the ignition off and back on appears to give me back the power that is lost as I hit the red zone. Before I had all the new bits (rad,top hose, thermostat, inter cooler) when it overheated it would boil and bubble. Not any more. And I used to lose a lot more water. After yesterday's 30 mile trip I have had to put in 1 and half litres. Before the new bits I would only lose cabin heat as it went into the red as there was no water. Now I have cabin heat or sometimes not and going into the red previously would terminate the journey as no coolant left until I filled up with coolant again.
Also I hear gurgling water noises that I've not heard before. Not sure where as they are usually heard not long after starting car. I can never trace them.

When I take off coolant cap and bleed cap to top up there is some pressure released. And the coolant level drops to about an inch below the join on the middle of the expansion tank. This appears to be not much coolant lost but then I can actually get almost 2 litres in before it comes out of bleed.
A few more bits of the puzzle!

Cheers

i have heard that to bleed the system properly. you need to unbolt and raise the expansion tank up higher.

check the return circuit into the tank...when the engine is warm do you see a clean flow back to the tank on the return pipe?, make sure that you dont loose coolant/pressure on the return circuit ... put a new tank cap if it wasnt replaced lately...and make sure it's well tightened

xat44 said:

i have heard that to bleed the system properly. you need to unbolt and raise the expansion tank up higher.

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The tank is not bolted in place - just plastic clipped.

Check for water pump leakage. This isn't always easy to see because the pump is low on the engine, but you should be able to see water lying on the s..t plate (with a torch that is).