Progress!!

Turns out the impellar on the water pump was spinning uslessly on its shaft when the engine got to temperature... fine when cold but once warmed up the coolant wasn't circulating as it should! This could have been done sooner, but I was told there was nothing wrong with it! The continuing process of elimination meant it could only be the water pump after swapping the head. So all's well with the support car now, though back-up (a 4x4 this time) awaits collection in Surrey... so looking forward to the new year to get back in shape.