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There could be other reasons for your problem. As Kano Nordi said, a blown gasket will often show up as bubbling up in the radiator if run with the cap off but cold.

If the head gasket has been done recently it shouldn't be that unless the head has become warped. The torque on the head nuts should also be checked after 100 miles or so.

I'd check the thermostat and radiators first as advised by John. When my big bore Nordwest was giving gasket problems it would pressurise the cooling system if given the beans even before it started to run hot.

The bike is still only 95% complete and I have only run it while it is sitting on my workbench. I didn't want to fully rebuild it before checking everything. I still need to check the thermostat. do you have any tips for filling/bleeding the cooling system?

There is a devious hard to spot, unless you know where it, bleed bolt on the head, look for (I think) M6 bolt not holding anything on the head, there were photo's printed on the German forum recently if you like to check

Indeed there is a bleed screw.

An air lock in the sytem will certainly cause overheating as I found to my cost on GFR#2. Not my fault as the level in the expansion tank was fine! I suspect the previous owner had refilled the cooling system but not bled it properly so it was missing a very significant percentage of the required capacity.

There is one screw the one on the right hand side where the hoses come out of engine (and to the right of the spark plug) is there another one i have missed?

Hi Stu that's the one