Sport Coolant not circulating

Hi all.First time here. I just purchased a 2007 3.6 v8 diesel with overheating problems.
I have changed water pump and thermostat and still the same. So removed both thermostats and still coolant not cicirculating. Bottom pipe cold and top one boiling.i have bled the coolant so many times now it still will not circulate.i have also bypassed a valve box under the passenger wheel arch.water from a hose pipe will flow into rad and out of thermostat pipe and also the way round but when I connect pipe back to top of rad it stops circulating again.i have been told it might be head gasket problem. If so why would the coolant not circulate

Blocked radiateur ?

There might be a small blockage some where and the waterpump is not strong enough to get around it.
When I put the hose pipe through the rad it flows ok.

Check your egr coolers

EGR cooler blockage, IIRC there is a small inline thermostat above the larger one - have you checked / changed that? Additionally if you vin has 7A before the final six digits, there will be one in the pipes for the EGR coolers themselves.
Have you tried vacuum filling it?

Mepps said:

Try Removing the radiator cap and pour water into the radiator... water should come out the bottom at the same rate you pour it into the top. If none comes out, the radiator is completely blocked. If some water comes out, or it comes out more slowly, it is probably partially blocked.

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Also try a hosepipe on the heater matrix lines

Update. I think it might be headgasket after all.when full of coolant it is bubbling when engine is running,it boils at a later stage.when I open bleed screw it constantly let's air out.
I can't think of anywhere else where air can get in constantly
Thanks for all your replies so far.