I need to model one for my 1914 BEF for The Great War.
I'm afraid the Leach catapult wasn't introduced until March 1915 (supplied by the department store Gamages of London), so it isn't any good for a 1914 force. Before that all you have is a myriad of different improvised designs constructed at the front, such as 'pipe guns' made from iron water pipes, one end of which was buried in the ground and which fired improvised jam tin bombs. Don't have any details or pictures, I'm afraid.
But if you want one anyway, what scale figures do you use? There are actually two different Leach catapult kits available in 28mm. Great War Miniatures do one with a Western Front crew (available from North Star) and Woodbine Designs make one with a crew dressed for the Dardanelles.
Another pic of C.P. Leach's catapult:

Another option for a scratch-build is Captain West's 'Spring Gun', which fired Mills bombs and other projectiles:

Both of these came into use in 1915 and production of them ceased by the spring of 1916, as better options such as the Toffee Apple mortar and the Stokes had rendered them obsolete.