I drove Maltby's Farina bodied Aprilia NGP 50 many thousands of miles when I worked for him. It was a great car and had the most superb gearchange. I helped Bob West rebush all the bell cranks (when "photographic" business was quiet) and it was probably the best steering column change I have ever used. You could change gear with one finger. The roadholding was more predictable than ordinary Aprilias and it went faster, probably due to its shape and weight distribution.
At one stage, Maltby put EGF's engine in it, which was a particularly good first series motor, which made it a great car for long distances. Maltby and I went on a job to Scotland in it and he was done for speeding in Glasgow and cautioned by another policeman further north (I forget exactly where). There was an old "hill climb" alongside Lock Lomond which we went up and down a few times, just for the hell of it! That was my favourite Aprilia.
I don't remember it being particularly noisy, but it had a useless handbrake and on one occasion, Maltby and I drove it back to London from Weymouth without any brakes. We waited 'till it got dark and the traffic thinned out a bit, but we made it without hitting anything. One of the dodgier drives of my life! Happy memories, John