I think you will find this is a No 29 artillery trailer or limber. for the 18 pounder. The No 24 was longer than this one and was the first limber for the 25 pounder, though normally seen with 18/25 pounders. However, this gun is a Martin Parry conversion (to pneumatic tyres) of the old, pole trail, version of the 18 pounder, so no way can it be an 18/25!
Everyone knows the No 27 limber, seen on almost every 25 pdr pic, but WWII saw also the Nos 24 (25 pdr), 26 and 30 (4.5 How), 29 (18 pdr) and another for the 3.7 How whose number I have not been able to work out. Some were just pneumatic tyred versions of the horse drawn originals (like the No 29).
Interesting pics. I wish you could see the gun behind the limber on the FWD!
Chris