Why not use the AR's fire control group? Timney makes a triggers makes a self-contained upgraded trigger unit that just drops in place. It would need only two pin holes for the hammer and trigger pins and the specs for locating those holes are already all over the web. The only other dimensions you would need to account for are the pocket to clear the body of the trigger and the trigger slot for the booger hook. Using an AR style trigger will also allow you to incorporate an AR safety as well by adding a couple extra holes in the receiver. The hammer that is meant to hit the firing pin could be set up to hit an archery trigger type release.
So what I'm thinking is this put into your receiver...:
http://www.brownells.com/rifle-part...s/ar-15-drop-in-trigger-module-prod40120.aspx
and then the job of actually holding and releasing the bow string done with something like this...:
http://www.ebay.com/bhp/bow-trigger-release
With a cantilever to transfer the AR hammer strike motion to the string release mechanism. If your receiver is thin/short enough, you might be able to do away with the AR parts and just use the bow release mechanism with an extended trigger lever.
That's all I got off the top of my head and is in no way the only way to do this. I've worked on plenty of guns and a few compound and recurve bows, but have about zero actual crossbow experience to go on. I'm not sure how the commercially available crossbows do it (re: the trigger and string hold & release mechanisms) so please take that into consideration before you go buying any parts.
EDIT: I just noticed you are located in the UK. I don't think the Timney AR parts will be available to you as they're over $100 (USD) and they are export restricted above that price. You could possible get the standard Mil-spec AR parts in a lower parts kit under that price ceiling and use that if you still wanted to try the AR trigger.
Here's just one of of many of the AR-15 Lower Parts Kits (LPK) available online... (the price is about average from what I've seem of them lately @ around $65 (USD)):
http://www.brownells.com/rifle-part...parts-kit-ar15-sku100011594-55088-115948.aspx
As a bonus, the LPK also has all the safety parts and a pistol grip included if you wanted (or are allowed) to include those parts on your crossbow.