Have been trying the Y&Y training plan but despite walking 20 minutes every day and then eating a packet of Jaffa Cakes I'm not losing any weight. Is there an advanced version or should I try the triple packs of jaffa cakes?

cut out the walking

try biscuits rather than cakes.  The extra cooking cycle adds calories.  However, you save calories because of the no VAT rule for cakes.

this is also the reason why Calzone is more calorific than ordinary pizza.

Some basic diet rules.
      1. If you eat something and no one sees you eat it, it has no calories. 
      2. If you drink a diet soda with a candy bar, the diet soda cancels out 
      the calories in the candy bar. 
      3. When you eat with someone else, calories don't count if you do not eat 
      more than they do. 
      4. Food used for medicinal purposes NEVER counts, such as hot chocolate, 
      brandy, toast and Sara Lee Cheesecake. 
      5. If you fatten up everyone else around you, then you look thinner. 
      6. Movie related foods do not have additional calories because they are part of the entertainment package and not part of one's personal fuel. 
      (Examples: Milk Duds, buttered popcorn, Junior Mints, Red Hots and Tootsie Rolls.) 
      7. Cookie pieces contain no calories. The process of breaking causes calorie leakage. 
      8. Things licked off knives and spoons have no calories if you are in the process of preparing something. 
      9. Foods that have the same color have the same number of calories. (Examples: spinach and pistachio ice cream; mushrooms and mashed      potatoes.) 
      10. Chocolate is a universal color and may be substituted for any other food color. 
      11. Anything consumed while standing has no calories. This is due to gravity and the density of the caloric mass. 
      12. Anything consumed from someone else's plate has no calories since the calories rightfully belong to the other person and will cling to his/her plate. (We ALL know how calories like to cling!) Remember, "stressed" spelled backwards is "desserts."