Update 8th April 2020:
More got done today (after gym time and the unblocking of the utility room sink's drain (awful job).
Didn't get out to the "shop" till the afternoon and have had a good "rummage" and a "dabble" with the front half of the trike.
I did get the BB tacked on and fitted up with cranks. The position is good, the changes have put the BB in the right spot at least.
The mech-post is obviously at the wrong angle, and this will be corrected, along with a host of other changes.
Surprisingly, there was no interference with the frame, but then the BB wasn't in the centre of the frame.
Here is a shot of the very tight clearance of the chainrings with the frame.
At one point I put a chain on (loosely) and the chainline was NOT PRETTY. This persuaded me to do a bit of frame chopping and BB shuffling to get some better alignments.
So I have chopped a section out of the drive-side of the frame and reinforced it with 3mm plate in order that tomorrow I will be able to shift the BB across and improve the chainline.
However, with the BB shell tacked as "dead-centre" as I can measure/judge it; the chainring now strikes the cutaway frame....Go figure!
It must have been WAYYYY out of line earlier.
While it is beginning to resemble a "dog's breakfast" it is also getting closer and closer to completion.
If we have another wonderfully sunny day tomorrow I shall persevere and try and get the BB shell properly welded in, and in a good orientation (square & true) and the chainrings on and the mech-post set at a decent working angle.