Thanks @Richard_Phillips. I think you are absolutely right and that the conventional wisdom seems to be that you should insulate before investing in any fancy heating technology. It is generally cheaper and as you said, any heat pump will be fighting a loosing battle unless the property is properly insulated.
I’ve actually parked the heat pump idea for now and am working on insulation instead. We have already had the double glazing (wooden frames) worked on as they were very draughty. I’m also working on adding another layer of insulation to the upper floor.
I have already asked the community What is the best insulation to use? and Are consumer-grade thermal imaging cameras any good for spotting heat leaks in a house? and the responses have been really helpful.
Thanks again for your input.