To provide a conclusion to this thread, I searched around online and found a bunch of bikes with the right Stack/Reach measurements. Most of them were "endurance" or "gravel" bikes in the 58cm/XL range. I exchanged emails with Mark over a few of them, and I ended up trying out and then buying this one (in 58cm). I didn't go with Evans, because this bike has full 105 with shimano hydro disks and a carbon frame for under a grand - nothing Evans had came close. I then went back to Take3Tri to get fitted onto this particular bike. We set the saddle flat, the height right, and then started moving it forwards - it's about as far forwards as it will go! Then we put the adjustable stem on and tried it lower/higher and longer/shorter. I ended up moving one 1cm spacer from below the stem to above it, but leaving the length alone.
I've been riding it around a month now, and I'm loving it - it feels really planted and I can climb seated much more comfortably than I could previously. The only quibble is that there's the occasional zing zing noise from the brakes over rough ground - if it doesn't go away on it's own then I can usually make it go away by touching the brakes, but it's till irritating me - I need to service it properly and see if I can cure that. Oh, and there are none of the Cube specific mudguards (that actually fit the fixings on this bike) available, so for now I'm using crud ones instead.
Cube by
duncancmartin, on Flickr
