Rear brakes and discs replaced at under 18000 miles as faulty. Now has 22600 miles and I need front ones done, but not under warranty this time. Should I challenge? Car is 2.5 years old and have also just had alloys replaced (common fault of bubbling, so they said).

Its certainly worth asking the question but generally front discs and pads do wear more heavily than the rear ones.
Not sure what the terrain is that you most often drive (e.g. motorways is fewer braking, but heavier when doing so - town driving lots of braking but mostly light, etc)