As you have found, the only Geo Johnston* dying in Scotland in 1987, aged 61, died on the Isle of Bute . Could he have died elsewhere or his death been elsewhere? Other ages within 2 years of that given are 59, Rosskeen and 59, Aberdeen. There is one aged 41, Park Circus, Glasgow (in case the headstone is wrong).
The nearest deth reg that I can find in England is Jan 1987 in Portsmouth, with a birth date of 17 Nov 1927 (George Douglas Johnstone )
Gadget