OFTEC registration isn't mandatory - https://www.oftec.org.uk/consumers/heating-installation-work-and-building-regulationsIf it's leaked very much this is an insurance job. It's the plumbers insurance issue if you contracted somebody in to fit it
Thier is an industry body for oil fuel and you need to speak to them. Your engineer needs to be registered
Its more serious than one might suspect usually. A few drips isn't a big one but I suspect this was a fair leak and proper repair could run into big money
Sorry to bear the news but it dies need more than a good scrub if its got down into the ground and your concrete.
That's what I meant. Wonder if it effects his insurance ?OFTEC registration isn't mandatory - https://www.oftec.org.uk/consumers/heating-installation-work-and-building-regulations