Thanks Lee, I will pass this on to my garage to go a bit further on the diagnostics as you have mentioned. I will also try driving the car and braking round the local car park this weekend when it's clear of cars. Good tips and thanks again.
I have just been on the phone after leaving my wife's Yaris (03 plate) with our garage. Same problem with the ABS cutting in for no reason. All checked over but still happening. The garage has another Yaris just in with the same problem. He's asked me to drive it around and will look at it next week to see if he can fix it. The problem is trying to find which wheel is causing the problem. I will keep you posted.