when getting new tyres is it better to change all 4? someone told me if you don't it could **** up the diff?

 

Then that 'someone' is talking bollocks. The diff's job is to allow different wheels to rotate at different speeds when necessary. Why would different tyres break it? Perhaps if you drove around with 19" rims on the left and rollerskate wheels on the right, you might work the diff quite hard, but that's it.

As a matter of safety, it's a good idea to have all four tyres showing the same amount of wear and the same size/brand/tread pattern. Which is why people rotate their tyres from one wheel to the next quite often to even out the wear. My Scoob for example, eats up the n/s/f tyre fairly fast because I have a lot of roundabouts near me. It might grip a little less and not perform so well in the wet, but I just swap it with the offside tyre. Eventually of course, you end up with tyres that are fairly evenly worn all round, which is then time to change them.

A cheaper option is to replace the front pair or rear pair at the same time.

 

cheers for that