Sounds like fun Tom!
Mike,
you can pick up a fairly decent late Great War deac for a shade under £400 at the fairs but don't expect it to have matching serial numbers or to be anything exotic.
The first add on the Milweb deacs page is offering what looks to be a large selection with mag cut offs for not too far over the lower end of the price scale, whereas the seller in the link below has more sought after offerings that are at the higher end of it. Whatever you settle on, make sure your confirm exactly what you are buying and have it looked at by someone who knows a bit about them to make sure it is the beast that it claims to be.
http://www.battleflagmilitaria.co.uk/shop.php
Ry