Virgin have done it for ages and on each occasion i've told them there's
no way i'm paying for it and they've ended up giving it to me for
nothing - this is without any arguing/pleading either

Do not understand this, FIRST ?
The airlines I use for a long time been charging extra for the extra leg
room seats in these positions.

I was sat next to one last month in an ordinary seat, it was empty but I
was informed to use it I would have to pay by the cabin crew.