Turns out, when the customer had bled the radiators, the gravity fed system wasn't refilling.
Simple enough so far.....or is it?
The f&e tank is stuck in a completely inaccessible space above the flat.
So, with my 'thinking outside the box' hat on, I installed a semi-permanent filling loop between the cold mains and the flow pipe in the airing cupboard. This then allowed me to top up the system from the bottom. The customer is now warm and cosy, and I'm off to swap a toilet in Borehamwood