1.5-litre or 2.5-litre?
I'll go with the 2.5 as I guess this is supposed to be a sports car and the 1.5 would prove a bit anaemic for the purpose.
The base car was probably an RM-series Riley saloon dating from c.1948/1949.
I have no idea when or who built it but I guess it was probably in the 1970s or perhaps later.