Had a response today. It's not just a £500 payment which I kind of hoped; but they will reimburse me up to £500 for the fitting of an aftermarket alarm (while stating that they cant supply one, nor fit an aftermarket one, nor even advise who I can approach to install one).
Going to have a think. Not sure how reliable aftermarket systems are, and how much work to get them fitted. Don't want to be left with a
Phoenix Nights style alarm which I can't shut off, not have to deal with spare fobs, etc. Really I'd just want a proper, incorporated alarm...you know, like most car makers fit :/