Problem replacing heater core

1990 Disco

I've got the heater out OK but cannot split it to replace the core.
According to the Haynes manual I should be able to slide the lower edge
of the inlet flap through a gap between the blower motor housing and
outer case while separating the two halves. Problem is - there isn't a
gap so the cut-out in the inlet flap holds the two halves together as it
goes either side of the blower motor housing.

I've spent over an hour trying all ways and now hoping someone can give
me a hint because I am out of ideas.....

The label on the heater is:
RANGE ROVER
BTR 0410

TIA
--
Martin Beenham (martin dot beenham at hotmail dot co dot uk)