Was changing a rear tyre on my corsa, so I checked them all and the front one which is badly warn on the inside (came from front driver side), ive swapped with the spare (good tread), but now ive got 2 tyres, one which has a pretty cracked sidewall, even though it holds air fine, but it only has about 1.9 to 2mm of tread. Whereas the tyre which is badly warn on the inside has about 5mm of tread everywhere else!

I should be able to get another one tomorrow but whats best to use in the mean time?

Badly warn outside but good tread?
Or tatty but legal tread?