Got it in front of me now.
says
TURBO SYSTEM TYPE: F68 (first 3 digit of my reg too!)
SERIAL No:2731 MOD.
FITTED BT: BSM DATE 8-2-89
SERVICE INSTRUCTIONS:
SPARKPLUG TYPE : STD
GAP 0.75mm
IGNITION SETTING-
BTDC AT : STD R/M
IDLE CO 6 % 5000 RPM
OTHER DETAILS-CONSULT
OWNERS INSTRUCTIONS


BLAH BLAH BLAH.

The XR4x4 fusebox lid seems most likely now