Hi can anyone tell me why the radiator water seems to be going into the header tank after filling the rad up and running it the temperature gauge seems to be up and down all the while have changed thermostat and put a new rad cap on does this sound like head gasket

yep sounds like HG is it bubbling into the bottle?

Will check tomorrow

What he said

What they both said....

I have seen some people mention that the egr could be over pressuring the systems

if the EGR cooler has gone porous and failed then it will, yes, pressurize the system and cause a problem like you have. However, this is seldom the case, more often than not it's the head gasket that has failed. EGR blank kits are available on ebay and the like for around a tenner. Check for coolant leaks, bleed any air out of the system, change the rad cap and all the basic stuff first, after that the only thing left is the head gasket. Temp should get get up to temp and sit exactly half way regardless of towing or going up hills etc, if it goes above half way you've got issues you need to sort out. I am assuming yours is a classic L200 circa 1998 to 2005. Have you had the truck long ? quite a few unscrupulous owners will sell their trucks when they overheat and the next poor unlucky sod is left to sort out somebody else's mess.