Cyberbeej,
I am planning on fitting a winscreen, (although I am a long way from actually doing it!), and did a little research on getting one made:
I found a automotive glazing company close to me (
http://www.windscreendorset.co.uk/index.html) and sent them an enquiry. That is, a picture of a Seven style screen and the logo of the screen marking required by IVA (as mentioned above). I asked if I sent them a template whether they could make something to order in laminated glass with the marking and to the shape of the template.
Answer was yes, not a problem at all. I didn't enquire about the price though.
So, suggest you find some where similar and local to yourself and see what they can do for you. Like the firm I conntacted they initially looked like an Autoglass place that only did chips and production cars, but further on in their site they mention bespoke glazing and have pictures of pretty old cars. If you do find out a price could you let us know, even if it's just a ball-park fiugre. I've really no idea how much it might cost, £100, £200 more/less, who knows?
Hope that helps,