When I worked at REME workshops we did some vehicle restoration work for the Royal Artillery museum at the Rotunda and after the workshop closure, the Curator put some work my way to get some of the vehicles on display outside in to running order. At that time it was impossible to get the larger exhibits under cover, that was, until the new museum called Firepower opened on the old Arsenal site. There was never a large footfall to the Rotunda and I suspect income was not high. The exhibits inside were exceptional.
There was a large parking area next to the Rotunda building and it was full of artillery, vehicles and related equipment, including AA radar trailers and an Auster AOP.
I remember the 7.2 in Gun as the staff referred to it as the 'Spike Milligan gun', I was told he donated some money towards it.
regards, Richard