An ASHP would cost me 5 or 6 times more.
Possible, but only if your current heating system is shoot.
You must have had a complex oil burner, if it needed to be 'pieced back together like a jigsaw'.
No, just old, with broken bits not available as spare parts any more. Apparently they could be stacked back into the case to make it work, but there was a risk that if they snapped too much more when taking them out during servicing, it may become impossible to stack them. It also made servicing take a bit longer and there was a minor risk that if it started taking too long then the firm would charge more.
You mention that your boiler 'approached 30 years old' - isn't that a persuasive reason to get another ? Are you confident that ASHP will last 30 years ?
Fairly confident it'll do similar because I've had other refrigeration units last longer, like
@Tenkaykev mentions, and they work 24x7 all year, unlike the ASHP which only does 30 minutes a day for half the year. I also feel that we were fairly conservative with our ASHP selection, buying a Mitsubishi instead of one of the newer companies, because like so many here we had plenty of FUD about noise and reliability. (However, this wasn't without a drawback: it meant we ended up with an installer which was technically good, bordering on great, but apparently not so competent at business matters, now having gone into administration.)
It's also worth noting that the old burner stank, especially when we returned home after a holiday, and my asthma has improved since getting rid, so it probably wasn't working perfectly, although no servicer found anything wrong. It's only fair to note that a new oil burner sited where it would comply with the new regs would probably have also dealt with most of those problems, as there would be a more airtight door between it and the living spaces.
Our plumber says that it would be a straight-forward job to install a new heat pump - certainly no need for new pipework.
So how would it end up costing 5 or 6 times more?
Our pipes and rads are fine - for use with an oil boiler. Folks I know who have installed GSHP and ASHPs, have changed to larger radiators - an additional expense..
That varies from house to house. It's not always needed if the previous heating system was a condensing boiler actually running at a temperature where it can condense efficiently, such as
the 50°c recommended during the recent energy crisis, but some people decide to change them anyway because they're getting the plumbers in and the system drained anyway and bigger radiators mean lower temperatures mean lower running costs. We had most radiators replaced because some were rusting out, but it was mainly the living room and kitchen ones that were made larger: the kitchen had been extended and I suspect the person doing it guessed what radiator sizes and got it wrong because it was always a bit cold in there, despite having the boiler glowing away.
As mentioned by
@chris-suffolk, GSHPs (& ASHPs) cost significantly more than Oil burners. Obviously, if I had a brand-new house and a new oil tank and plinth for it had to be installed, that would increase the price of an oil-burning system; however, it would still be much cheaper than installing a GSHP.
I can't make the numbers add up for a GSHP retrofit, where you have a much higher installation cost and lower running costs. They probably only work out financially in new builds, where the groundwork kit is on site anyway, and in suitable ground.
If heat pumps are 'a lot simpler', why are they so much more expensive ?
ASHPs are not much dearer like-for-like (avoid comparing dumb timeswitch+thermostat burners with a modern heating system that gets rid of ever going "oh it's a bit cold, let's blast the heating"), but there are still economies of scale and fewer good installers, plus the costs of putting right mistakes made by housing developers (like installing basic insulation, or replacing tiny radiators that have to be run too hot to allow condensing burners to condense with appropriately-sized ones) seem to get blamed on heat pumps. Those are other things that Ofgem or the housing regulators really ought to be tackling, instead of throwing braindead grants at the problem and ignoring that some cowboys then just put their prices up by the maximum grant amount.
