This may already be known (I can't find anything searching), the Sun Holidays booking site seems to be available to everyone now, without any codes.
Taken from the Website -
Great Family Holidays From £9.50 Per Person!
To check out availability and book your break, simply click on 'Book Now'.
Click on an area to check out the facilities at each park and for full details of any Service Charge or Entertainment/Clubroom pass charges that may apply.
We're sure you will find your ideal break, and hope you'll soon be enjoying a fabulous Holiday from £9.50!
Dates available in April, May, June, July, September, October and November.
Hope this will help someone.