This sounds like a large project and there are some oddities. Financial contribution? Tender for a domestic heating system?
Is this engineer paid/employed by you- if he is then why haven't you sacked him?
It's a bit complicated. We're agricultural tenants and the engineer is employed by our landlord. I am aware that under normal circumstances (I'm also a landlord) that the landlord takes full costs of such an installation, however ours is not a standard residential tenancy so an agreement has been made for the landlord to contribute so much and us so much. I think this is where this engineer is struggling as he's used to dealing with projects for tenants where they have no financial contribution and as such what he says goes (if that makes sense)
And yes, if this was my property, he'd have gone long ago.