westy04 seems to have got a good deal. eBay is worth a scan but double check any BIN prices with a web search – several times I bought new goods cheaper away from eBay.
Not wanting to put you off but would an alarm actual give you any benefit? That is, if you're out and the alarm goes off are your neighbours going to respond to it? If it's purely a visual deterant then a blank box and sensors would be just as good.
My little group had a wake up call two weekends ago when we were camping in Wales. Someone who had turned up for the National DH Champs had five bikes nicked – we estimated about £15k worth.
Personally, I prefer a suptle approach to the external security measures. You need to look closely to see the 'improvements'. Internally I have a combination of different lock types and anchor points.
I had considered an alarm but decided a quality cctv camera (inside) would be better – simply because if the alarm did go off then no one would respond to it if I were out.