If you look in the grooves of the tread, about every 12" round the wheel there is a little block of rubber in the tread. These are wear indicators. When they are level with the surrounding rubber, the tyre needs changing. As Ethel says, its 1.6mm on a car, but on my truck I can go down to 1mm over 3/4 of the tread width. It always seems weird to me that trucks can have less tread than a car, but thats the law for you!
Thankyou sddsddean and CHuckFickens,as I said to Ethel this is my first MOT on my VW Golf and when I had it serviced last July I had my the paper from that service on what they did and also telling me that my tyres were wearing down and needed to be changed on this years service plus tracking had to be done,I did check my front tyres this morning and they were down to 2mm.
As Chuck says, its best not to wait until the last minute and if your front tyres are down to 2mm, I'd get them changed this week (especially as you have a front wheel drive car). Go to a decent tyre fitter (word of mouth recommendation is best), but please don't go to bl00dy Kwik Fit! They are just robbing bast@rds.