The requirements are set out here and include

(c)[the heat pump] replaces the heat generating components of the original heating system installed in that property (where applicable), other than any—

(i)supplementary electric heater, including any immersion heater,

(ii)circulation pump, or

(iii)solar thermal collector.

So the legal question which arises is whether the gas fireplace is part of 'the original heating system'.  That's an interesting question!

Our wood burning stove wasn't removed and I would argue, if challenged, that it's not part of 'the heating system'.  If that argument succeeds then it should also succeed for a gas fireplace.  

I would require them to reinstate and if they refuse to do so ask them point you to the legislation that requires them to remove.  They may well cave in at this point or direct you to guidance, which you can reject on the grounds that it is the primary legislation that matters. 

James 

Whilst I'd love to agree with @jamespa, I think it's at least rather ambiguous.

The eligibility criteria state in the legislation that:

5.—(1) For the purposes of these Regulations, a property is an eligible property where—

(a)it is a building,

(b)it is not social housing,

(c)if it[has had any heating system installed, and was, or will be, first occupied or used]before the date on which the relevant eligible plant is commissioned—

(i). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

(ii)the system providing space heating or water heating, or both, of the property prior to the installation and commissioning of the eligible plant (the “original heating system”) is—

(aa)fuelled by fossil fuel(1), or

(bb)an electric heating system, and

[(iii)there has not been a previous grant from public funds for a heat pump or biomass boiler at the same address, and

(iv)there has not been a previous installation of a heat pump, or biomass boiler at the property as a measure promoted by an electricity or gas supplier for the purpose of achieving their home-heating cost reduction obligation under section 41B of the Electricity Act 1989 or section 33BD of the Gas Act 1986, and

(v)there is a valid energy performance certificate for the property],

A woodburning stove may provide space heating but it is not fuelled by fossil fuel and so could be argued - at least within the provisions of the legislation - to not be a part of the original heating system. On the other hand, a gas fireplace does provide space heating and is fuelled by fossil fuel so I suspect would be more difficult to argue against being part of the original heating system.

I hasten to add that I'm not a legal expert and so my educated guesses are no better than any other lay person's educated guesses.

As an aside, though, this raises an interesting question about how an installer is supposed to deal (legislation-wise) with an open Victorian fireplace. They were designed to burn coal - a fossil fuel - and most certainly provide space heating. The typical method of dealing with this is to buy a Chimney Sheep to block up the chimney in a way that doesn't risk a build-up of damp but it can easily be removed again repeatedly at will so isn't a form of "capping off". Is the installer supposed to cap the chimney off? Or remove and destroy the grate? Or brick up the fireplace? I'm dead certain none of these actually happen, but I wonder what the law theoretically requires.

Coming back into the realms of practicality again, is your gas fireplace one that gives a "real fire" look and feel similar to a woodburner when active? Or is the gas fireplace purely a functional heat-emitter? If its primary purpose is decoration and heat is just a byproduct, I'd argue that it doesn't form part of the original heating system, but if its primary focus is generating heat then it'd be difficult to maintain that argument.

Bearing in mind I have no expertise or prior knowledge, a cold reading of the following OFGEM installer guide:

Section 3.20. talks about evidence requirements.

The first bullet refers to a valid EPC which will show the prior heating system "a previous EPC for the property that lists a fossil fuel or electricity as the **main heating** fuel" (emphasis added by me).

On my EPC my gas boiler is listed as "Main heating" while my wood burner is listed as "Secondary heating", what does your EPC say about main and secondary?

I would conclude from that, that provided my main heating had been disposed of, and otherwise met the BUS rules, that would be allowable.

But please do your own research, I know not of what I speak.