I'd be concerned about the "nominal sum". That might cause some issues if it were challenged. Why not ask them to buy the share at 5% of its market value? -- then you have a thousand or so quid of deposit to boot. If someone were offering me a nice deal like that, I'd be only too happy to lay down some capital of that magnitude.
(I'm not familiar with the rules on Permit aircraft, but presume that the same rules on PT and Aerial Work apply as if they were Private Category C of A aircraft. Is that right, Genghis?)