I used to know someone who made tyres and what he said made a lot of sense to me. He said tyre manufacturers spend huge sums testing and designing their tyres to work in pairs so if you buy a matching pair you know they will work as they were designed to, when you need it. If you mix tyres they may be fine, but they may not be, and you wont know that until you need it when for example you do an emergency stop.
Personally I always buy an matching pair every time and stick the part worn on Ebay they always sell.
I nearly always buy from Openeo - great prices and great service, that was after a recommendation from Trev.
