Thanks guys, tapping in at the headlight it is, by the looks.
I have a Narva plug and play loom that I had on another vehicle that was easy to find a trigger for under the dash, but clearly that isn't happening on the XJ. There is enough trigger wire on the Narva loom to feed it back towards the front.
I have checked that FSM wiring diagram again, puzzling over why what should be the high beam wire is always live though... clearly that isn't the right wire.
If only someone made Xj specific plug n play headlight looms with driving light wiring inluded.....
Realistically you should be able to tap into the wiring near the steering column and run it through a switch out to your relay.
If it were me and you are going to tap into the headlight wiring at the front I'd be running it through a fuse (say 5a close to the tapping point) to be on the safe side.....
Success! I don't quite know what wire I tapped into the first time, but I decided to have another look by taking off the panel under the steering column (and the steel knee guard).
A lot easier to see. The loom to the left of the column has a red wire with orange tracer. That is the high beam feed and works as a trigger for the driving lights.
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I jammed the control wire in the fuse slot of the LHS High beam fuse 
does the trick 
