Hello Kim,
Very sorry to hear of your misfortune, and it's a distressing need for an introduction into the forum. Let's look at this from a back to basics and the cost becomes logical. Autoglass will repair and replace your car glass if it is available, but I doubt whether convertible hood rear screens comes under that heading - probably with most makes. Audi becomes the next port of call, and because it's a request for a likely slow moving part for a 7 year old model, then the price is going to be sky high. Well you told us that, but at least it makes some economic sense, so you now have to look for economic alternatives.
Impossible to fit a new rear screen into your existing hood? I seriously doubt it's impossible.
Useful contacts would be classic car trimmers - they are used to making bespoke hoods and should be not be frightened off by this. Google searches should give you some contacts, but try approaching local classic car clubs for all important recommendations. We have the old established Jack Smiths down here at Swansea and you should be able to get his number (and recommendations) via. Lotus (I think it is ) forums. If you fail let me know. I know it's a long way from Surrey, but you could have a free weekend in Mumbles on what these boys charge in relation to others! They are seriously cheap in comparison, and I believe they also do work for insurance companies.
Other alternatives will of course include scrap yards who may just have a rear screen from a salvaged one with normally vandalised roof fabric. You could do searches via. Partfinder - I think it is, but there will be others.
This misfortune could work in your favour if they do write it off and you buy the salvage, but be aware that the car's subsequent value will be less and of course, bang goes your NCB despite it being of no fault of yours. Not on, but true.
I hope you are able to sort something out Kim. Let us know how you get on.
Kind regards,
Gareth.