I have used the cheapo ones for the last 8 years, sometimes they dont last but you cant be certain a £17 one is genuine or will last, better to buy several cheapo ones and throw them away when they have had it, they are accurate enough if your water is below 10pmm even the genuine ones only claim +/-3 pmm.
The genuine ones will need a battery swop after 18 months so Cheapo copies work out better value by far,