Did this job today and worked first time. What I have ended up with is the timer switches off the outdoor units and leaves power to the indoors (they all go in to fault, but the pumps still run. The indoors have power for 3 minutes before the power is cut to them too. Thereīs a 4 minute delay on the timer, so if you put credit in during the 3 minute "pump time" the outdoor units wonīt power up again before the indoors switch off clearing the U4 fault.

Thank you to everyone who helped me with this, it ended up being rather simple in the end.