Update. Car had to be towed as I couldn't get the !@#!@$ lock ring off
Anyway, pump was pulled and it tested fine, no problems. What appears to have happened is that the EFI hose has weakened significantly where it attaches to the fuel pump outlet (and pressure is the greatest). To the point where it is expanding to approximately the size as my radiator hose
As the restriction in the fuel hose is removed, the pump is unable to supply constant / sufficient pressure.
I've only had the pump in for four months, and the lines were inspected when it was installed so it looks like it has degraded gradually over time due to the larger capacity being output by the pump.
Has anyone experienced anything like this before??