Ahh that is very interesting devotion, I was unsure of the Halfords trailers as I had seen some not so good reviews of it; especially about the stroller wheel attaching to the tow bar bit being a bit awkward.
have you tried Amazon? My childminder got a double one with a handle so she can push it like a double buggy from there for £100. It would prob only be able to come tomorrow though if it's coming directly from Amazon and not through someone else but its worth a try.
[http://www.amazon.co.uk/Bumper-Adventurer-Duo-Trailer/dp/B0011HXJ36/ref=sr12?ie=UTF8&s=sports&qid =1245928949&sr=8-2 here] £99 free delivery will come tomorrow if you order in next 2 hours. (there's others too on there but not all delivery tomorrow)