Robert
Not sure with your layout but if you follow the pipework from the cold water inlet, you will find a drain tap. I think it will be either blue or yellow. The lever of the 'tap' will be lying flat to the floor, you need to lift this little plastic lever to its vertical position which will allow the pipes and boiler to drain through the floor. You should also open all sink and shower taps to avoid air locks from holding water in the system.
Ideally, you should have done this before leaving your last site, this would have completely drained the system.
Don't forget the loo flush tank either.
Al
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