Right I have done a lot of asking around today about this and have been told that it's not against the law to have same tyres on the same axle say 205/50 on the fronts and 205/55 on the back. I also asked some mechanics seeing as I was already there if it would knacker the diff with the sizes of tyre's, they said not at all even with AWD. They said you would need to have a 14" wheel's on the front then 17" on the back that would kill the diff but not the tyre's, they said even though there wider and does have more rubber on the ground they will still move at the same speed at the same time as the front's. It's just the alloy's that would if they was 2 different sizes :D
God I need to get a life ;)