Sorry, tought the post was about a MK VI, but it isn't so forget the following text an bolt the doors shut or modify the locks to MK IV or MK VI.
Rob, the MK VIs i saw had burst proof locks below a cover that made them look like old Wilmot Breeden MG/Cobra locks.
The doors, if opened and looked from the rear, show a "V" shaped notch going deep into the width of the door where the also burst proof catch/striker plate slides into.
A Wilmot Breeden lookalike cover with actuation lever just sits on top to access the modern lock.
The MK IV door lock and striker bolt look very familiar maybe in the MK VI it is again Audi or VW.
I'm pretty shure behind the chrome cover in the there door is a more or less modern production car door latch.
Both parts should be out of a production car, adapted and covered to get this old Cobra look.