I buy it from the Philip Kingsley website - a litre is £52 (!) which lasts me six months. You get rewards points and after two years I just got enough for a litre free.
I was using the PK silver conditioner too, to help my colour. It has lasted for ages and smells of lavender. I find the other PK conditioners too perfumey but like the no scent one.
I do use regular conditioner too - any sort. I only put it on the ends of my hair so doesn't seem to affect my scalp.
I am just waiting for someone to come on here and tell us about a PK dupe that is half the price! Anyone?!