(Yeah not really what I was asking was it?)
Apache is working fine and serving 5 other reverse proxy apps. So I think the problem is probably Calibre using some non relative URLs or something...I had the same results with one other app and this was the cause in that case (hardcoded references to /gui/). Fixed the non relative URLs and it worked fine, with that app.
Anyway, it's no big deal, and in any case, I've decided the reverse proxy thing actually has some negatives when you use a bunch on inter-related apps anyway, so I have unwound the changes and am back to serving Calibre on its own port.
Thanks for your attention in any case, and the great program that is Calibre.