Switch to Heat Pump

Just looking for a bit of advice. Keen to clean up our carbon footprint, I'm considering switching from mains gas to a heat pump.

House is a 4-bed detached, built around 1990. Just before we purchased the house in 2011, it underwent some renovations and interior wall insulation was added. BER at purchase was a C3, (210.73 kWh/m2/yr) on 127.1 square metres.

We have since upgraded the windows to triple glazing (2012) and I've doubled up the attic insulation by adding 270mm above the joists last year. We installed a closed-door (so reasonably airtight) solid fuel stove into the fireplace around 2012, but we rarely use that nowadays. Even though it's no longer an open fireplace, I've stuck a chimney sheep up into the chimney flue as a secondary draft barrier.

We added an extension in 2014, so floor space is about 145 sqm now and we also have double-glazing across the width of the house at the back. I wish I'd bit a bit more environmentally savvy when we did this extension, it's not a bad job but I do think a bit more could have been done around airtightness, etc, particularly at the joins to the existing building if I'd known what to ask for.

So, to the gas - we use it for dhw, space heating and cooking. We've reduced our usage from a regular 18500 kWh per annum to about 16000 last year, mostly through better controls on the heating (we had no boost on the rotary timer, so we'd knock on the heating for an hour when needed but leave it coming on for several days at that time until I'd spot it and tidy it up). We've got 10 rads + 2 towel rails in the house. All are on TRV's, and generally set to 3/5 unless it's baltic or we need to heat the house up quickly. Controls are non-existent, so everything goes on together with the TRV's being the only constraint. Upstairs is generally toasty, downstairs in the extension (kitchen/dining/tv area) is fine but loses heat quickly enough, front of house downstairs struggles but sitting room rad isn't working properly and hallway rad is aesthetically hidden (not my choice!). We supplement sitting room with a small electric heater when we're using that room.

Cooking we're considering moving to an induction hob anyway, with the aim being to get off gas altogether if we go ahead with the heat pump.

The boiler itself it at least 10 years old - I think it pre-dates the renovations before we bought by a couple of years. It is a 24kW condensing boiler, so 90%+ efficient.

Immersion has gone bust on our tiny (like 100-120l) hot water tank - there's five of us in the family so that need a major volume upgrade anyway. The boiler is grand but no spring chicken, and I'd hate to replace it like-for-like by virtue of a failure and being under time pressure, so I'm thinking of making the switch to a heat pump sooner rather than later.

So the main questions for me are, 1) would we be close enough to heat-pump ready to make a go of it? I don't think major renovations are an option but happy to chase up small mods (e.g. chimney sheep, insulating velux blinds for extension, etc.). And 2) any advice/considerations on retrofitting a heat pump would be welcome. I realise it may not be cost-effective, but we do have some upcoming repairs, etc which offset the cost a bit.

Thanks in advance!