I've been having similar conversations regarding some of my cars as the insurance is due on them all this week and current insurers wanted trackers on 3 of them and a level 2 immobiliser on the 1961 Flaminia.
The Lancia was the easiest to get a current valuation at what I feel is a reasonable level as pretty much every one on the market or recently sold were pitched about £30k above the value I have on mine. The Vanquish more difficult as values softened a bit last year. However the cheaper ones really are potential money pits. Probably not possible to get one in the condition mine is so the specialists valued it right at the highest level for a Vanquish.
The GTN is always the awkward one. Highly unlikely to be replaceable unless one of the other owners could be convinced to sell. I suppose you could get a good Series 1 and try and locate a 4.6 engine and do a homage but that will still end up expensive. Anyway, here's hoping the insurance is always a waste of money!
The good news though is I have all 5 sorted, business use on all of them so never an issue as to which car I might jump in and what I'm doing, without the need for any additional security over that factory installed when new, agreed values where relevant and a decent premium.
