Years ago the register used to offer a letter supporting the owners valuation as realistic (based on the photos etc etc, the value probably represents a fair estimate of what the car would fetch on the open market) which wasn't a definite valuation and left plenty of "wiggle room"
I suspect that due to the lack of definite firm value, it may not always have been as helpful as some people would like.
I think it's probably also worth reminding people that an insurance valuation is very different to a sale valuation. To value the car for sale, you would have a price (17k mk1 aside) that you could realistically sell the vehicle for. In the case of cars that have had extensive restoration or modification work, this is likely to be a small fraction of what has actually been spent on the car. Whereas a valuation for classic insurance should be a figure that, shouod the worst happen, would see you able to replace the car with another of the same model and bring it to the same condition.
As an example, if you buy a clean, straight and mechanically sound car for a grand, then have the interior retrimmed with suede, fit a nice stainless exhaust and a few other fun bits, it might then only sell for 1500. Yet you have spent 7k on it, so your insurance agreed value would want to be around the 7k mark.
Another thought on insurance values, if you don't take the piss and claim that your 100k miler mk2 is worth 17 grand, photographs and details of all restoration and modification work should be sufficient when accompanied with receipts and decent quality photos. I haven't ever been challenged on the value I have put on any of my cars that have been on classic cover until the current GT2 I've got, when the insurer thought my valuation was a touch on the low side. And before you get all excited, no I won't tell you what the value was.
Just make sure you get decent, flattering pictures that [b]haven't been edited. [\b] Don't wash the car where you're about to take the photos, the floor being covered in bubbles looks bad. Try not to take pictures when it's dull and overcast. And if you don't have a decent camera, ask on the forum if there is someone who does have a decent digi camera local to you who knows how to use it.