Thanks for info.
There is a setting to prioritise the type of road preferences - unsure if only between motorways and other roads or down to A road / B road levels - might be worth a look.
Apologies if you've already tried - guess you have reading your mails !
Other work around is to set way-points avoiding the route - ie select point(s) along route you want it to take or enter points to avoid - the previous owner of mine did this I noticed.
Hope it helps in some way.