Just bought a scooby and want to check the water level in the header tank, where abouts is this on the engine.

 

Suggest you get yourself a handbook for the car. Try Alan @ AS Performance, Birtley, Tyne and Wear. 0191 410 3770.
The water header tank is on the driver side of the car, towards the front of the engine, mounted on the inlet manifold, connected to the expansion tank by a water pipe and funnily enough it has a radiator type cap on it.