I dont think this is clutch related Mark @mudski. It seems to be further down the line... Like there is play between the gears somewhere in the diff or gearbox or something...
I should actually almost call it a "jolt" as the car just about jolts forward when something "grabs" and begins applying momentum to the car. But it happens "after" clutch engages not during... If that makes sense it takes a second or milliseconds even , I'm confident its down the line...
I'll come around when i get a moment you take it for a spin you'll see, feel and hear it trust me lol.