A cautionary tale.

Look and learn.
Beware the walking Freezer.
NOT "walk in"  - actually walking.
Hopefully it may help somebody else.
We found we were being forced to defrost practically weekly. Quite early on I accurately diagnosed air ingress. I also, inaccurately, diagnosed the cause.
I/we thought the seal had gone.
Saw "The Man" (NOT that one Sue) ...
Nah, it's your door, I've never known a seal go on one of those.
He should know - he did. 

Apparently the whole of the Fridge/freezer had somehow wandered backwards to the extent that the Freezer door and Cabinet door, which ought to be both:-
A) Parallel to one another.
and
B) About 2-3mm apart.
were in fact,
C) Emphatically NOT parallel.
D) Varied from mms apart to about 1.5cm apart at the hinge end.

Cure - tug the entire unit forward about 3/4" - Sorted.