SOLVED! Looks like I’ve finally found the problem. The intank mesh fuel filter, although looked spotless, wasn’t letting much fuel through. So when the car was sat for a while, the pipe between the tank and the pump slowly filled with fuel so the car would run for 10 minutes, however not enough fuel was coming out of the tank, causing the pump to whine and the car to bog under acceleration.
I took out this old intank filter and the fuel flowed like it should. I’ve installed a new filter before the pump and the car runs perfectly.
Thanks for all your help over the past few months! Onto the MOT now.