With regards to whether a cracked lense is an M.o.T failure, it depends on the severity. The book says that the light must not show white light to the rear, even then depending on how bad the crack is, you could go to Halfords or such like and buy Lense repair tape which basically looks like coloured sellotape. As a tester, this is what I suggest for small cracks and it is the cheapest option rather than buying whole lamp clusters!