I think you’ve nailed it! Blew it through and there was definitely something in the way. All back together and the pipe that was popping off now doesn’t. The temporary zip tie holding it in place is removed and the normal sound of returning fuel is back. The pipe from the HP pump doesn’t bulge anymore either. 
 

legends! Thank you.

 

hopefully it gets through the MOT now. The garage might be able to get me in today 🤞🏻

Fingers crossed for you  I had a go at fixing my gas analyser last night but no joy...  looks like it's going on eBay for spares or repair!

Ah that’s a shame. 
 

mill post back after the MOT which is today or when it stops raining - probably in August 🤣🤣

20 minutes ago, m1tch said:

Ah that’s a shame. 

The cell that measures hydrocarbons in it shows a failure code and when Mr.Snap-On engineer was here to maintain it, he informed it was about a grand to replace the cell. Considering I got the analyser from a friend who ran a local garage for a few hundred quid just to be able to map cars a bit more accurately in terms of MOT compliance (exactly as you are finding), and the fact that I've done that about 4 times in the several years of ownership of it, let's be honest.... it's not worth it....