I should have said that the discount applies to their club sites, who's prices are built on a pitch fee then per person. The smaller Certified Sites (5 unit types), tend to be a flat price per pitch - and are supposed to be ONLY available to members.
I've generally found it good value for money, but their marketing and website is run by 5 year olds....and if you try and engage them in a conversation to improve things, you soon give-up!
I've just (for a test) tried to book 3 nights at the West Runton Norfolk Club site and it's showing me £53 for 2 adults for 3 nights (grass no elec). For non members it is £71.45.
Member's saving is £18.45. Membership starts at £38? so 6.4 nights would see your membership recouped.
By non-club standards, the quality of the sites is very high, but don't suit everyone. (Lots of caravans, a few rules, but clean hot spacious showers etc).