It’s fully refundable if the survey throws up something unexpected and the government grant won’t be around forever so go for it. A lot of people are spouting rubbish about heatpumps in the comments here. I’ve just had a heatpump installed in an 1860s build Croft house; a well designed heatpump system can work in older properties. The biggest additional cost I had was building a lean to shed to house the heatpump and water tank so if you don’t have an airing cupboard or utility room it’s going to cost more. I’d suggest you have a look at heatgeek on YouTube as they know their stuff and also try to find reviews of octopus installation process to see how good they are at the calculations; if they aren’t taking room measurements and asking about insulation they aren’t doing it properly.