Well, it's been a long time, but it's also a long story. Here's an update.
The dealer inspected the car and noted that it seemed to be rather more rusty than expected. They put me on to Mazda customer services.
Mazda said that the anti-corrosion warranty covers perforation of the body panels from the inside outwards, and does not cover chassis/subframe parts.
I countered that I would not expect to find ANY rust on a new car. OK, it's not completely new - it's manufactured at the other side of the world, transported for weeks by sea, sat in a compound for a time, then transported halfway across the UK to the dealer.
However, Mazda refused to do anything about it.
So, my next port of call was back to the dealership. I got the same response from them. I then stated that I would not expect to find any rusty bits if I had bought a beemer or similar upmarket executive car. Mazda isn't exactly at the bottom end of the market. My expectation was that a new car should have no minor imperfections, unless they had been pointed out to me at the time of sale. Sale of Goods Act and so on. Reluctantly the Service Manager agreed and went to speak with his managing director.
After some time, they agreed to treat the rear subframe parts.
So, in November the car went in for a couple of days. (It was now 6 months old).
I lead a busy life, so didn't check immediately, so it was February when I was hosing the salt off the underside that I checked again. It looked like it had not been done. Phoned up the dealer again.
They read out the work sheet - remove engine undertray .... But the problem was at the rear. They'd done the wrong end of the car.
Back in again for a couple of days.
Six weeks later, I had another look underneath. They'd sprayed the subframe with a rubberised coating, but the spot-welds were still rusty.
So, I've just got it back a third time, and it looks ok - for now... Hopefully that's the end of this saga!