Undertaken some more diagnosis this morning.
Dropped the coolant and removed both of the bypass thermostat valves to check them.
Here's the first (as my car is a DSG)
Here is the second (turbo coolant line)
These look like this
And then opened up look like this
You will notice that there is a brass active element in these. The pin position on the one in the photo here is fully closed but i had to use pliers to get it there. Both valves allowed me to blow freely through them in both directions when I took them out of the car, clearly both brass active elements are shot.
I put the brass thermostats in a cup of boiling water and they did expand, but then when i cooled them down and tried to press the pin back in it was very stiff, way harder than the small spring would overcome.
I've ordered them from the local TPS, c.£24 each inc vat.
I also tested the new temperature sensor against the old one (took ohm readings at various temperatures) and they are basically the same, so the old one was fine.
I've re-assembled the car with the old bits for now as we need to use it, but I've manually closed the thermostat pins so at least for the first warm up cycle i'll be able to see if it really has solved the slow warm up issue - I'm pretty certain it will have.
Through the research I've done it's very interesting to note that the cooling system design changed mid-05.
http://volkswagen.workshop-manuals.com/golf-mk5/index.php?id=5042and then to
http://volkswagen.workshop-manuals.com/golf-mk5/index.php?id=5043Spot the difference? Yep, no bypass thermostat for the turbo coolant pump line on the revised one. It's still there for DSG however.
Early 05 manual cars = Main thermostat + bypass stat for turbo cooling pump
Early 05 DSG cars = Main thermostat + bypass stat for turbo cooling pump + gearbox cooler bypass stat
Late 05+ manual cars = Main thermostat only
Late 05+ DSG cars = Main thermostat + gearbox cooler bypass stat
In short, just make sure you buy a 06 plate manual GTi :)
This thread here is also worth a read, many people seem to have the same problem and overlook the small thermostats.
http://translate.google.co.uk/translate?hl=en&sl=nl&u=http://golfgtiforum.nl/forum/viewtopic.php%3Ff%3D12%26t%3D1582%26start%3D100&prev=/search%3Fq%3D4e0%2B121%2B113%26start%3D10%26client%3Dsafari%26sa%3DN%26rls%3Den%26biw%3D1572%26bih%3D883