The price of Sierras is definitely on the increase. It may be worth advertising this on one of the owner's forums (e.g. XR Owners Club or even RS Owners Club).
Wasn't that long ago you couldn't give these away, often seen advertised for 5/6k, escort cosworths too, now look, makes me wish I'd bought a few and stored them.....
That's your insurance; other policies vary (mine makes no representations about the car needing to be insured by the owner for my third party "driving other cars" cover to apply)
Towing or trailering it to the garage may be the cheapest option, or insuring it and then cancelling within the cooling off period. Taking out rac/a cover is another possibility, as is (as others have mentioned) handing the keys to a motor trader who has "any car in his care" insurance