I’m insured through Adrain Flux’s, actually both my cars are through them and both are guaranteed value.
The RS was very simple to get the agreed value. I requested the agreed value when asking for a quote, they accepted my valuation upon receiving photos of the car and proof of mileage. It cost and additional couple of quid, around £10.
My 300zx was a bit more detailed. I needed to still do the above but I also had to list all mods along with the price to do the work. I also had to get a report from a specialist I use on the car (however I think any reputable garage might be able to do this). For those of you with modded cars and valued in the £20k this might be the process you need to complete.
I really do rate AF. They’re not the cheapest quotes I got for both cars but their customer service is second to none.
P.s. Sorry for the highjack FootLong.