Oh, PS: I did try to ask a local bike shop's fitter/owner if I could pay her for some of her time to advise me on this, but her response suggested two possible services they could provide neither of which seem to quite work (1. Translate from another bike that fits me - See nature of my problem, above; 2. Full fitting - seems a little premature but something I will undoubtedly do if it's worth proceeding with).