fatbloke - Belated congratulations on the MOT.
I'd agree with
Mr T that the primer is the main give away.
If you are saving up for a professional paint job, why not give it a “temporary” coat of paint.
I posted these photos of a hand painted car on my own build thread.
Total cost =
£35 including the brushes!
A slightly left field suggestion would be to paint the hard top in a completely different colour.
E.g. Body work in red and hard top in black.
That way your lorry driving friend may think you have put an MX5 top on a Ferrari roadster to keep the rain out.
Good luck, Paul.