As the Mio support staff couldn’t see the fault on the two models they had tried in the office, I started off by sending my 405 in for repair as that was still under warranty. It has come back and I was told the main board had been replaced.
I have now had a test ride and can say that the fault is still there. I don’t there that there is much doubt that this is a world wide software fault in Mio’s devices as three out of three units have now been doing this.
I have emailed them this and am awaiting a response.