A couple of you have been really helpful in uncovering this vehicle's past. Thank you.
One or two also wanted to see some pics. This Matador has been with the Chatham Historic Dockyard since the early 80's, when it, along with some artillery pieces and a searchlight trailer were gifted to the museum. It was often used as a portable stage at 40's period events since, but has suffered in the past few years, including an incomplete restoration in 1995 and external storage during lockdown (don't ask).
We are now attending to its rot - both metallic and wooden - and trying to get it back to full working order for the Yard. And yes, I know it should be RAF Blue/Grey. I'm sure there will be more pics of the horrors we find over the next few weeks. If anyone has a pic or diagram of the battery box/location for RAF drop-sides, I'd be really grateful!