Ok, so swapped it this morning (dead easy with an oil filter chain) and I have mixed results.
When not using the brakes and the ignition on, after initial pressurisation I didn't hear the pump click on in a 10 minute period. Then got bored of waiting.
Before it clicked on every 4 minutes so improvement there.
Next, when braking, pump clicks on when being used for the third time. So I now get two pumps without the motor kicking in. Was clicking on with every brake application before so again, good improvement.
But, after being sat for 2 hours when ignition turned on, it primed for 20 seconds so its still loosing pressure. I assume it shouldn't need to prime at all following a short rest?
I haven't bled it yet but assume this isn't the issue.
Someone mentioned about potential for a duff one way valve. Is that the next likely thing to change?
Anybody know where it is? Or is it integral to the pump?