Hi all, an update:

Replaced most of the vacuum pipes and the fuel return lines today. I don’t think any were actually leaking but they were all in need of replacement. Did not cure the poor running though.

Then set about checking all the wiring for the diagnostics socket. Sadly I did not apply the KISS (Keep It Simple Stupid) principle here before pulling out the centre console and stripping back fabric wrapping to check the wires – turns out it was just a sodding fuse! (Thanks Dad!) I hadn’t realised this would have been fused as I thought it was run from the radio loom and would therefore be covered by that fuse.

IMPORTANT LESSON everyone – check the simple things first!

Anyway, the diagnostics has thrown back:

Code 01237 – Fuel Cut Off Valve N109
Code 00553 – Mass Air Flow Sensor – G70 (I did say it was a cheap code reader otherwise it would probably have said Air Mass Meter or AMM!)

I gather the Fuel Cut Off Valve could explain the revving to 2000rpm on start up but does anyone know if this could throw back a false reading on the AMM as well? It’s just that I did try replacing the AMM with a genuine part and it made no difference at all to the running. But there again, I wasn’t aware of the fault with the Valve at this point so maybe it wouldn’t have done anyway.

Anyway, searching other threads leads me to believe best option for now is replace the Fuel Cut Off Valve and take it from there.

Thanks to everyone for comments and suggestions so far and if anyone else has anything else to add, please feel free!