You fix some, you break some
Front and rear brake problems are now solved. The rear brake was solved by the idea of running a full outer housing for the cable. The tube guides didn't do the job unfortunately, but I managed to find some plastic wall plugs that had exactly the right inner diameter to accommodate the cale housing, and exactly the right outer diameter to wedge it into the hydraulic hose guides nice and tight! (pictures to come soon). The front brake was fixed with a lot more fiddling of the positioning of the caliper's mounting position on the forks.
My chain. Oh, and various bits of skin on my leg and foot. After fixing the brakes I took the bike for a gentle Sunday afternoon ride with my girlfriend in Virginia Water. After a few hours riding we were returning home when the chain suddenly snapped on me. It wasn't the 'gold link' - as I first thought - but another non-descript link. To be fair, it may have been one of the links that I fixed no when I shortened the chain. Luckily I have some spares from hen I did this so I'll try to do it a bit better this time!