The simple answer to your initial post is:-
You were not happy with the installers choice of condensate pipe run.
A reputable GSR would know exactly how to ensure condensate pipe complied with current standards.
His mate, an OFTEC registered installer pointed out a problem with the pipe work. He has attempted to lighten the situation by complementing the GSR installers work. Clever, he has made sure he will get more work off the installer.
The problem you now face is the original installer will not have issued with any relevant reports. Doubt if he will arrange annual service on the boiler either, so possibly no 2nd year guarentee .
CD10 Installation report. No self respecting OFTEC engineer will commission work where CD10 is missing. If he is daft enough to do so, he is signing off and taking responsibility for whole system. Brave or foolish person, you decide.
TI/133 Oil storage risk assessment. Comes in 2 parts.
First covers enviromental Hazard and the second Fire hazard.
As you said
It was fitted by a Gas Safe registered engineer, he doesn't do oil really
so therefore you do not have any certificates relating to the installation. I doubt if the OFTEC registered installer bothered to leave you a copy of his CD11 service/commission report either. If he did, no doubt you would not realise what it really means.
I can guarantee you that if we inspected the installation the first fault would be the oil tank.
Just to make it easier for you to understand the problems you now face.
A few years ago an oil tank split on an Anglesey property.
Shame it was not installed correctly. Total cost for decontaminating area was in excess of £11,000
plus the installer had to install a new tank foc. Cost was not an issue with customer but the inconvenience and mess was an issue.
We were called in to carry out service on the boiler 'coz previous installer had cocked up. Carried out tank risk assessment and guess what, the tank was installed exactly the same as the one that split, resting on two lintels..
The potential risks were explained to customer and his signature obtained and copy of T1/133 was left with him. He has now take the responsibility for the tank. We still service his boiler and every year he has the same report regarding poor tank installation. When it splits again it will be his responsibility to have area cleared of spillage.
There use to be a detached house not far from us, it burnt down a few years ago.
Tank installed tight against house wall. There was a fire in close proximity to the tank. Part of facier and soffit dropped on the tank which melted and contents leaked on to paths that sloped back towards house.
The fire was accelerated due to the amount of burning oil standing on paths. Conclusion, no house left.
Insurance companies are experts at wriggling out of paying out, and apparently both incidents were not covered by the insurers.
Employing trades by choosing cheapest will eventually come back and bite you.
Cheap installers are like fitting budget tires on a high performance car, premature failure.
The potential risks and breakdowns seem to have been passed directly on you.