Rather expensive for a 15ml bottle (£15.95). The fellow who valeted my car told me about a local company called Orchard Autocare:
http://www.orchard-autocare.com/
They have a product called Hydrophobe:
Same principle as the Gtechniq stuff, but a damn site cheaper, £15.95 for a 500ml bottle. It's also a lot less time consuming applying it... just spray it on!
I did my windows the week before Christmas and it works a treat for me. The car hasn't been washed since then and is pigging, but the windscreen surround (where the wipers don't get to) is just starting to show signs of dirt sticking to it, whereas it would start showing after a week or so. Hope to get it washed this week and then I'll find out if it still works.
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2001 A8 D2 FL 3.7 Q - LPG Conversion, Engine: AKC, Gearbox:FBB, Colour:LY5X Aqua Blue pearl effect, Interior:WJ light beige/dark beige valcona leather with burr walnut inserts
My DIY jobs -
Replaced alarm horn,
Replaced Coolant Temp sensor,
Updated RNS-D firmware,
Installed Reversing Camera and Digital TV,
Dashcam Installation,
Retrofitted Rear Blinds,
Auto-Dimming Rear View Mirror,
Chrome Boot Struts
Sold! 
2008 S8 D3 5.2 V10 - Engine: BSM, Gearbox:KLW, Colour:LY7F Suzuka Grey Pearl Effect, Interior:Black/Black with Silver/Black valcona leather with carbon atlas inserts, Bang & Olufsen, Heated windscreen, Soft-close doors, TV, DAB, AMI, Quad Zone, Auto Boot, Auto dim Mirrors, Keyless entry, PDC with rearview camera, AFS II Headlights
My DIY jobs -
Fan Jet Windscreen Washers,
Retrofitted Electric Side Blinds