Job done!!
Yes, there's some coating on it and I'm not sure if it's aluminium or steel, and under the pipe/tube cutter blade it felt like heavy anodising.
You can pull and hold a vacuum on the injector line now with the throttle at 100% and the pump line plugged. My DZ170 has a slow air leak under the same test.

I'll hold back on the bottle of Moet & Chandon Brut until I've had a few more successful trips to the flying field, but I just returned from the field this afternoon fingers crossed that the engine keeps running that good. Smooth idle at just over 2K and smooth run up to full throttle and back down. Fingers crossed (everything crossed) this has solved it !