Hi Sally.
I’ve actually covered the costs from our first year in a separate post since then, you can read about it here:
Just quickly to answer your question - heat pumps operate at a lower temperature and so they can be better at heating a room by giving off more energy if they are slightly larger. However, we found that because we had already replaced our radiators with new triples, we didn’t have to replace any. Plus, it’s the sort of thing you can do later if you find any rooms underperforming or cold.
My follow up post should help with any costs questions, but suffice to say with the green grants the heat pump should be a much cheaper option than gas; at least if you get the right installer and your house isn’t enormous.