Many thanks to everyone that has commented on this question. I hadn't really thought about the issue of sub woofer position within the room but on reflection (see what I did there?) it's not surprising that it would have a significant effect. Therefore not wise to constrain positioning options by building it into a cabinet. I shall proceed accordingly.
Although I am an electronics engineer turned computer/telecoms person by profession, most of my electronics activity has been at radio frequencies, so low frequency audio is rather mysterious to me. I was, however quite amused when I worked out that the reason all 16ft stops of one particular note sounded much louder than the neighbouring notes was that one dimension of the room was a precise half wavelength. It demonstrated that the room is actually quite high
Q, which was a bit of a surprise.
Lots of new things to learn here! Thanks again.
John.