James,
I too had a CB400N Superdream and have to confess the only problem encountered was "dead" fuel in the float chambers if the bike was every left for a few weeks. Long shaft screwdriver was the trick... used it also on my CX500 to deal with stale fuel. Latter remedy was to turn off the fuel cock about 1/2 mile from home and leave bike running whilst I washed it after the run home from work... left float chambers empty. No further problems.
However I think you must remember that whilst Honda engineering is super and I am sure the vendor did a good restoration... it is an old bike and uses older technology so expect the odd idiosyncrasy with her
Enjoy the bike. Do NOT oversize the tyres, especially the front. Keep to the original specification for tyres as they do not like the larger, more modern profiles.
At least your warm southern UK weather will allow you to use your machine whilst some of my Aberdeenshire friends are snowed in!
Rennie