In need of something that will cope with larger sprocket, my short cage is too small.
Ideally needs to be a 105 but would be happy with Tiagra.
Only other problem is I need it by Sunday morning.
Sorted. Mods can delete this now if need be.
For the benefit of anyone who lands here from google:
the 105 part you would have needed is the RD-5701-GS. The older RD-5700-GS increases the max chain capacity but not the max cog size (it's 30t on the older model, 32t on the newer one). Shimano don't sell a 105 10s road cassette with a 32t - you could go with an XT part, I used an SRAM PG1070. To get this setup to work, you need to turn the B screw on the derailleur most of the way in so that the der runs clear of the cogs.
(from having downgeared my bike for a loaded tour of the Alps)