Hey both, wanted to keep this post up to date for other UK purchasers.
So it seems almost impossible to find a AC to AC transformer in the UK, after getting a second transformer online that was AC to DC I ended up walking into a Maplin store to pay an outrages £20 for a AC to AC adaptor:
AC 24v
1500mA
Please to say this works great, but im not that happy with the quality of the transformer – its very large and makes a loud buzzing noise.
Im going to be in US for work next month, so going to order a Rain Bird UT1 Sprinkler System Timer Electric Transformer from amazon: http://www.amazon.com/Rain-Bird-UT1-Sprinkler-Transformer/dp/B000DCN8LS/ref=sr_1_1?ie=UTF8&qid=1375565707&sr=8-1&keywords=sprinkler+transformer
Maybe a DC model of the OpenSprinkler system could be produced for us UK people 😉