Ive only skim read the responses, so apologies in advance if this has already been said.
I think that the issue about what could have or would have been said or done is irrelevant at this time. I really do sympathise with your position tigersmum and I dont think it is easily resolvable.
My instincts would also be to take the moral high ground and leave the ball in their court, BUT I strongly feel that it may strengthen your position and give you better peace of mind if you take that stance following a dialogue (or attempt at) with the in-laws.
It is possible that the reason they have decided to renege on their agreement is likely to be based on assumptions that they have drawn. You stated that they declined your daughters offer of involvement in the plans. Could it be that they have assumed or interpreted the honeymoon presentation as YOUR daughter obtaining her wishes at THEIR expense? And perhaps more painfully, at THEIR expense, but in YOUR (and your DH's) name? I can kind of understand that they may feel put out that the gift was from you and your DH and no the parents collectively. Particularly if there was nothing to indicate that THEIR significant financial input. Please dont take this as an accusation, im just trying to be objective, so its just a thought.
Maybe they have rationalised their decision to themselves on the basis that they have and will continue to provide on-going financial assistance (hence the reference to the boiler), whereas they may view your contribution as a one-off?
Do they know that the total bill exceeded £13k?
I think that unless you and you DH are able to speak to the in-laws, nothing will actually be established and ill-feeling on both sides will continue to fester and possible escalate. I think meeting up with them will give both sides the opportunity to get things off their respective chests and enable the opinions on both sides to be aired and discussed. There is still chance that you may still disagree, but that's not the point im trying to make. If they STILL refuse to honour their agreement, I think then that is the time to take the 'moral high ground stance'.
Sorry to go on, but I just think that this could escalate into something really ugly and both sides may sadly end up hurt. Badly. If your daughter and her husband are blessed with a child, what may happen at the christening? What will happen at Christmas? will there be a determined effort to keep both in-laws apart? I just feel that in these situations its important to attempt to UNDERSTAND where the other party is coming from, even if you dont actually AGREE with it.
Hope that makes some sort of sense. But I say this coming froma large family where these kind of situations have dominated inter-sibling conflict.