I wish you well in your search, having judged the concours at the 60th there are very few cars that can be truly considered concours, Magnette owners tend to be users and enjoyers of their cars, and therefore tend to personalise them in some way, which moves the cars from the concours field.
For instance disc brakes, 5 speed boxes, flashing indicators etc.
I suggest that a possible solution is to find a car with really good bodywork for say £8 -£11000 and then sort out the cosmetics, there are a couple on ebay which may suit or Car and Classic. There is a green ZB at £14k(over priced in my opinion) in Salisbury area which looks a reasonable basis, needs the engine bay sorting and the whole of the interior, say £10K.
Motorbuild have two high priced Magnettes but both very poor quality,not worth the trip. There is a sound basis ZB in west Wales 07043226395 may be worth a look
Good luck
Trevor