Sites for £15

Hmm, I've just had a look at the directory and although it mentions discounts on pages 30 and 31 I can't find anything either, looks like you have to look for the dark red disc on particular sites. Not very helpful. 

7 sites for £15 in England,

Bromyard Downs Club Site, Herefordshire
Broomfield Farm Club Site, East Sussex
Cadeside Club Site, Somerset
High Onn Club Site, Staffordshire
Ilfracombe Club Site, Devon
Nunnykirk Club Site, Northumberland
Slinfold Club Site, West Sussex

2 in Wales,

Aberbran Club Site, Wales
Gwern-y-Bwlch Club Site, Wales

and two in Scotland,

Altnaharra Club Site, Scotland
Clachan Club Site, Scotland

https://www.caravanclub.co.uk/uk-holidays/special-offers/tour-for-less/

I think the Op has pointed out it's the directory which is lacking in detail unless I've missed something when I had look this morning. 

I only read the OP and a few posts Brue but suppose that any offers of £15 site night might only apply to first of the two years covered by the book

Probably be £16 next year anyway...

They could still have listed them and added an amendment in the supplement, should one arise.