When should I change my tyres?

I’ve been told that the legal minimum tread limit is 1mm. Should I let the tread go this low or should I change the tyre sooner? I don’t race my bike, it’s only used on A and B-roads with very little motorway use. I have checked the tread tonight and I have 1.6mm of tread left, I just don’t know if I should let the tread go right down.
Andrew1608, MCN forums
 
Once a bike tyre has got down to a tread depth of less than 2mm the lovely rounded profile has probably flattened out considerably and you’ll notice it in the handling. You’ve also got a much greater risk of picking up a puncture.
Unless you are flat-broke I’d change the tyres at 2.5mm of tread, especially at this time of year when we’re on wet, greasy roads and the tread pattern needs depth to channel away the volume of water lying on the road. What price your safety?