I wouldn't bothered to go ahead with this. Because at the end it will be long hassle and arguement and it not worth it to get all stressed out. ...
I'd do as baby_frogmella suggests and accept the offer, and from what you've said in the past it appears you can afford it!
As baby_frogmella says, if you've got a signed contract and they renege on it you could take them to court; but if you don't want the hassle then don't bother arguing your case, and just accept their refusal to complete the deal at the agreed price.
So nothing to lose really - you may get your FTTP(oD) installed at a bargain price, and if they refuse then you don't have the possible stress you're concerned about because you just accept their refusal.
Win-win.
FTTP 80/20 Mbps