Last time I did this, I went to Vintage And Rare on Denmark St. When it came to renewing the documents a few years later (valuations were valid at my insurer for 3 or 5 years), I took along the documents and they simply re-signed and -dated the same forms, as if to say "same value still applies".
V&R have moved out of Denmark St now, the premises is now occupied by No Tom Guitars, http://notomguitars.com/ - and they have a valuations service, see http://notomguitars.com/pages/valuations. If you use them, let us know how you get on.