need some advice or help on this one, a friends 315 started over heating when we were last out, we were doing a long trail ride with long sections climbing and he was losing power and over heating so we topped the water up etc but this kept happening. 

When we had time we check the water pump, impellor looked fine and turns with the kick start and isn't loose or anything, cleaned the rad and flushed the system through, refilled with fresh coolant. Still overheats so next thought was head gasket, whipped the head off and gasket looks fine but we replaced it as a precaution, still overheats so the last thought was the rad cap spring being a little loose with age? He couldn't get a new one they're obsolete now apparently so got a used one form a trials breaker and no better so now we're kind of stuck. Nothing else on the bike has changed and when you start it, up until it overheats it feels fine, pulls well etc.

Anyone any advice on what to check or try next?

Is the fan coming on?

Hot running can be caused by weak mixture or retarded ignition timing. Unlikely on a trials bike but blocked / restricted silencer can cause hot running.

Hose collapsing inside or split in such a way it impeded flow - unlikely but worth a check

Is the radiator really clean - remove it from the bike and hold it up against the light and look through it, you may be surprised how many fins are wholly or partially blocked.

Sorry shuld have said the fan was working fine, cutting in and out etc, so obvious never remembered that bit, the silencer could be something as the exhaust isn't in the east condition, otherwise mixture etc not sure but seems to be running, sounding and pulling ok generally until it overheats .

ideally we need another exhaust from a good bike to try and see what that does. 

Try the radiator caps from japanese cars at a scrapyard, they worked fine on mx bikes.