I bet your friend is very proud.
I phoned him up to see if he could come round & take a look. He lives in Gloucester & I live in Worcester, so its not just around the corner. We both worked at Autosleepers in Evesham a few years back where he did all the electrics & gas & I worked in the design department.
He actually thought it was a faulty APS & suggested that it was probably possible to replace without the need to strip the boiler down. The manual directed me to the APS where I noticed the kink in the tube.
Be cynical all you like, its just nice to know I have friends that are willing to help me out with being pedantic about it.