Just starting using my S2 that has just had full engine rebuild, pistons re-bore new cam etc.
Was pottering around in it yesterday, not going fast as running in. When turned off, you can here the coolant system, and the tank is quite pressurised. Is this common? Please help

 

The tank should be pressurised with a warm engine. What do you mean by hearing the cooling system?

 

you can here it bubbleing if you know what i mean, i wouldn't have been able to open the tank up or water would have come out. \but runs fine, starts ok, doesn't run hot etc.

 

Either you have a leak somewhere meaning that you gradually loose pressure to the point that the water starts to boil, or then you have a hot spot in the cooling system after shutting the engine (water flow stops). The most common cause of the problem is the close proximity of the turbo to the radiator.