The “correct” method should be call the concept shops, see if they have stuff lying about (http://www.specializedconceptstore.co.uk/), if not they’ll ring spesh UK, get a part delivered and the concept store (or dealer) sells you it. I found the concept stores were better than normal dealers as they don’t expect to get paid for the services. Thats not going to help you get out this weekend though.
I had a bag of bits with the frame, including all the spacers for the different fork options, but if you haven’t got those then its probably back to the shop. Unless you know a local machine shop who can knock you one up.
If the London concept stores are being dumb, try the Bristol one, they seemed really keen to help me with mine even though I built it up from parts. Though the mechanic did hint that the headsets/bearings were all custom “hard to find” things. Obviously they’d have to post it if they had one in stock, so again not good for this weekend.
If you have the 1-1/8 to 1.5 adapter and some spare forks you could try that? (grasping at straws here….)