I'd also recommend Triopair at Pontefract, near Wakefield - who also run a collection service, or if you live nearby you can drop it in and collect it.
The website is http://www.triopair.com/
email: info@triopair.com
and the snailmail address:
Trio Pair
Unit 5
Baileygate Industrial Estate
South Bailey
Pontefract
West Yorkshire
WF8 2LN
01977 708688
You can measure your existing awning at the end which goes into the caravan channel, in centimetres, then measure around your new van using a piece of string threaded through the awning channel (a tip is to make a loop in the end you're pulling through to and attach to something so it's easy to pull through - rather than trying to thread the free end of a piece of string). The difference between the two lengths is the piece you need to have inserted, and I think there is actually a maximum size for this insert. There's a guide on the Bradcot website which tells you how. http://www.bradcot-awnings.co.uk/products/measure.html