well folks, at last I've managed to get some signs of heat in my XJ-S.. after spending a frankly extortionate amount of money on a thermostat from Jaguar, bleeding the system numerous times, flushing out the rad etc, I finally spotted the vacuum controlled valve for the heater on the top hose...
Amazing what a bit of WD40 and a wangle with a screwdriver can do isn't it? Heater and A/C now perfect!
Just thought I'd post this in case some poor sod is freezing after coming in from his XJ-S and is frantically googling for a cure for the icicles..