Thanks for the replies and info;
Have now done my own check of tread depth and it is on average 7mm, which if new depth is 8mm, is wear of around 12%. Dealer who checked seem to have over-estimated wear, especially on the front tyres. Dealer's measurement of 6mm depth and calculation of 30% wear for that would indicate new tread depth of 8.7mm..
If anyone can confirm new Turanza 225/45 R19 tread depth that would be appreciated.
(I did indeed get a very good discounted price - Mazda dealers had incentives to move stock before end of November).
Car goes back to dealer next week to have second go at fixing the gear change problem, including test drive to show the problem before they touch it. I will report back.