Thank you all for your comments.
It has been a long standing concern of mine how the civilian world seem quite happy to place completely unsuitable pax in positions of significant importance on an aircraft and I do genuinely worry that when push comes to shove, they will be unable to fulfil the duties that go with their seating location.
I do feel that if someone offers to perform these safety duties for an airline, then to decline that offer (even at the expense of £30) displays a clear disregard for pax safety. And if that means that I question the ability of the little 90 year old lady's suitability for her occupancy of an exit seat, then so be it, I have no qualms about doing so. At the end of the day, the airline is asking me (and hundreds of others) to put my life in her hands, and that cannot be right.
Thanks once again for your comments, I might just take it up with the CAA.
Kindest regards
TKR