Runner SP125 Massive water pressure fault

Ok guys and girls
I've got this sp125 two stroke in with a major water pressure problem.
It's blown the same water pipe twice in 24 hours, the outlet pipe from the pump, and the water is getting shoved into the header bottle with some extreme force.
What i can tell you is that the rad is clear, so are all the metal pipes, i've had them all off and flushed and reverse flushed them all.
The thermostat is also out.
The way the water is getting jetted out the pump outlet makes me think it's the water pump that might be faulty, but then again it could be the head or barrel causing the over pressure.
It's not losing any compression when i leave the tester on it for an hour or so, so i suppose i could rule out the excessive pressure being caused by the piston/barrel/head. If there was a small crack in either of these, i would presume that it would show through loss of compression value over a small amount of time wouldn't it?
Anybody had these type of problems before on a runner 125?

 

Ok, so i can now rule out the water pump as well. Disconnected it from the back of the oil pump (he's running premix on this bike) and it's still spurting it into the bottle with some force. Even on tick over. Must be the top end then.

 

Head gasket gone or head warped, it can only be one of these, Steve.

 

Spent the afternoon changing the barrel, head, head cover, all the gaskets. Meticulously cleaned the mating surfaces. Used some sealant as well. Doubled up on the base gaskets, one mid size and one really skinny because I noticed that the cases were slightly out of line at the bottom of the cylinder.
It seems to have done the job. No hissing at the water bottle cap, water is going back into the bottle at a decent rate, instead of at force 10, compression is at 120 psi. Can't see any leaks from anywhere, so it might have done the job. Just finished it.
Customers taking it home tonight, lives about 12 miles away from me, so it will be a good test.
I'll let you all know.

 

Customer just texted, all good so far. Told him run it round for a couple of days and let me know how it goes.