Advice needed to change spyder heater core.

[droptop]

I am catching up with some overdue work on my spyder and need to change the heater core.
I looked in the Haynes manual, but the scant instructions seem to have more to do with an S2/ facelift model than my 1978 pre-facelift one.
I currently have the seats and centre console out but can't see how to start removing the heater aside from three seized philips-head screws along the front lower edge which are currently soaking in WD40.
The coolant hoses have been disconnected as I had to bypass the leaking core last year, so I just need to swap it and finish cleaning the interior and fit the s2 front seats while I get mine reupholstered.
Any help would be greatly appreciated.

[HFStuart]

IIRC you undo it from under the bonnet - four bolts / nuts around the air flap in the bit just in front of the screen.

[droptop]

Dashing out with spanner in hand and hoping it is that simple.
Thanks stuart.