Forgot to give update, clamp removed early next day. Thinking of getting "Private Property" signs but, to use the American legal phrase, that "assumes facts not in evidence" (i.e. clampers can read and will check and understand notices).
It is insured, SORNd and off the public road. DVLA have done this to be before. Today they also left a "Not Taxed" sticker on a neighbour's car, which is also off road and SORNd.
Usual way around being NFA is your letters go to the local benefits office, as that's where you get your money from. Though might not apply given the lady's lifestyle.
Not sure if allowed. Was on Google maps when this view came up:
https://www.google.co.uk/maps/@53.3435312,-2.2142906,3a,75y,270h,90t/data=!3m6!1e1!3m4!1sn5km3m7lvZmNN5YZZWORCA!2e0!7i13312!8i6656?entry=ttu