yes 18t is still available. £24.99 from any retail store. Cheaper online. Pop into a cyclesurgery branch or call one up and ask to check for stock. They will tell you which stores have them, pop in to one where its in stock.
If you can be arsed, cyclesurgery price match, so look on the internet and find a place thats cheap and has the item you want in stock (on a 2010 code) and just ask for the seller to check the online price for you. bish bash bosh sorted.
It'll cost you a bit more if you want cyclesurgery to fit it, and this time of year (summer) you might not be able to get it done straight away. Ask for availability when/if you phone them.
Sounds like you've just had experience of the worst of what shop floor staff in evans can offer.
Sounds like the dude who answered the phone just looked at the evans stock system and saw they didn;t have any in and just tried to fob you off because he was too lazy to do some actual work.
Sadly my thoughts exactly.