Just checked in Merseyside Av. Soc.'s "British Homebuilt Aircraft since 1920" (2nd Ed. publ. Apr 1979), and it says that John Isaacs began construction in 1969, and the aircraft flew from Thruxton early in 1975. It's approx 6/10 scale. I vaguely remember it turning up at one of the PFA rallies at Sywell, probably 1975/76.
Another scale/replica Spitfire design was produced by Marcel Jurca as the MJ-10. Jurca is probably better known for the Tempete and Scirocco, but has also produced plans for a scale Mustang. At the time the Homebuilts book mentioned above was written, two Jurca Spits were under construction in the UK. It would be interesting to find out if they have ever seen the light of day.