"The EPC talked about things I would not be allowed to do. Can't even get secondary glazing!"
The EPC still would have put that as a suggested upgrade on the certificate, irrespective or not of whether you're present or not. These guys have a bunch of boxes to tick and not ticking them is borderline impossible. They can't alter them to say 'listed building, secondary glazing not permissible.'
Anyway, I think your agent was out of order doing an EPC without your permission. No listed building has to have one; they can choose to, but they don't have to. Our local agent charges sellers £130 for them, so wtf did he think he was doing?
You do NOT have to have an EPC to sell a listed house, nor do you need one to rent one out either.
Having it could be doing you more harm than good.
Would you mind telling me what KwH is listed on the cert for heating and hot water, and how many m2 the property is? And do you think the m2 is accurate?
It's tough enough as it is with selling a listed property without folks seeing how much they have to give energy companies.
That said, people who like that sort of property generally already know about the issues with heating them and how much they cost.
Even so, I would consider insisting the EPC gets deleted, unless it's particularly flattering (which, in your case, would be a higher number D rating - but I bet you're nearer E39 or possibly even F).
You can get TRVs fitted on radiators, have a new boiler installed and install low energy lighting. That's about as much as you can do. Possibly floor insulation, but even that has to be done with great care in old buildings and you really, really, really need to know what you are doing in order to avoid damp problems later on down the line.