The MIAFTR system was put in place by the DVLA, ABI (Association of British Insurers), ACPO (Association of Chief Police Officers), and the Salvage industry. It is a point of law that we have to update it, and there are very specific guidelines as to how we do this - I have countless arguments with people over this sort of issue. We are obliged to enter every vehicle that was stolen or written off - once on, unless a mistake was made, the car will remain on there.
�5k is hell of a big drop - don't know what car you have there?
However, there is no case in law for diminuation of value - the idea of insurance is to put you back in the same position that you were in prior to the loss (i.e. repair the vehicle to it's pre-incident condition, or pay the market value in the event it is written off/unrecovered). This is all decided by the courts, not by the insurance industry. Obvously if we did have to take this into account, premiiums would increase as a result.
I sympathise with you totally - it's not a fair system, but it's a double edged sword.
Take the example - there are two seemingly identical cars for sale at a garage. Both are priced at �10000. One was a previous stolen recovered vehicle that has been repaired to an identical condition as the one that has not been stolen. Which one would you realistically buy?
The systems are in place to protect the consumer - I have bought plenty of previous ''proper'' write-off's (i.e categorised salvage), and have no issue with it. But some people will.
Insurance is, in general, flawed. I work in the industry, and cannot honestly say I agree with half the rules, but you have to work with it.