I would imagine if they didn't drain the entire system (as the thermostat is quite high in the system) then all they needed to do is top up via the expansion tank, as the rad would still be full of water (as it sits slightly lower than the stat) so minimal rad water waould have been lost....
 
Yeah, I think so - there was no need to drain the system, just took that top hose off, then the elbow joint! In hindsight I could have done it myself very easily, BUT the lack of stat may have foxed me slightly and I'm pleased they were there to double check if there was an overheating issue or not!
 

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