Assuming you checked for external damage (loose wires in the switch is the common issue, very easy to check this out), then the warrenty is good for 12 months each time you get the esc back from repair/ replacement so far as I understand.
I believe that the warranty period is 12 months from the original date of purchase. After this initial 12 months is expired the warranty is up. This is how virtually all warranties work. So if you are still in the original 12 month time frame all is good, if not it looks like you will be out the repair fee. If warranties kept extending themselves 12 months every time you got a replacement, they would go on forever.
Hmm, UK laws on this sort of thing are probably different to US ones- its all a bit of a mish mass of different pieces of legislations, and unless you are a lawyer some companies will fob you off with excuses or outright lie to you in order to avoid having to fix or replace something, I think this requires a Castle rep's input to be 100% sure either way...