I had my car towed by the AA for head gasket after my oil pressure warning had come on. They've done whatever they do and replaced/fixed the head gasket bits. They since said that my thermostat was faulty and i needed a new one. naturally i wasn't going to pay the £70 they wanted to do that when i know i can do it for less than £20. However since my red oil pressure warning light is still coming on after a while of running the car. It doesn't happen straight away maybe after 15 miles. I've done the usual checking oil levels etc and all seems fine. No oil entering the water anymore from what i can see. 

 

Is it likely to be anything to do with the thermostat being dodgy? Or any other suggestions greatly received! just want this thing working properly! 

Dodgy oil pressure switch sounds likely. Are you sure your head gasket had gone? As you may have been taken for a ride if the oil pressure light came on due to broken sensor

Well 'they sent it off to an engineer' and had all the bits replaced. I went to quite a reputable garage so hoping so and it definitely feels better to drive. Before they replaced it there was definitely water getting into the engine and oil into the water tank. I read a few posts elsewhere and people suggested the sump or some pipe might be blocked (can you tell i dont really know what im talking about here) however i would presume if they've stripped the engine that it would've been noticed and cleared?

They won't have taken the sump off to do head gasket, oil pick up pipe is in sump.

Hopefully for your sake its the pressure switch that's dodgy, if not get the sump off asap and check the pick up pipe is clear, if this is clear and the pressure switch is healthy then its either your oil pump is going weak, or your losing pressure in the engine somewhere, most likely the crank bearings, if it's them your looking at needing a refurbished bottom end.

Also it's not a good sign the light comes on after 15mins, as this means its coming on when the oil thins down when getting hotter.

The temperatur/coolant light has now come on. This day is just getting better and better!

Is the coolant level ok? Could be crud on the sensor in the bottle from the cooling system not being flushed properly. Mine had exactly the same after my hg went

Does not compute - low oil pressure indicates HGF? On what planet?