@CakeBoy - thanks for your kind words, it's so frustrating as it's probably something simple and I can't believe I've got 2 boilers with holes in the HE pipes. It's a brand new PCB, the problem was evident with the old PCB so I think we can eliminate that. The Flowmeter seems to be ok, having said that I've noticed that the 10 second flashing light on the pushbutton hasn't worked for some time (that's to indicate the pump has been running for 10 secs when back flushing) ... this may point to the Flowmeter being faulty. But shot volumes are accurate so I thought that indicated it was working and maintaining calibration.
Yes I have descaled the machine and it's very clean, I don't think there's a blockage anywhere and have removed and checked most components now. I've got a pressure gauge/portafillter and it consistently reads 9 bar, there is no movement on the needle at full pressure. There is no evidence of water leakage, no sounds of steam escaping, the boiler pressure is correct ... but somehow the brew water is getting too hot in the Group causing it to turn to steam