The emphasis is on keeping down the sugar in the diet. It doesn't have to be sugar-free, but you should avoid adding extra sugar, and should avoid cakes and biscuits with high sugar levels. The recipes section suggests many healthy alternatives to these. Sugary drinks should also be avoided.
Jam is a high sugar food, but provided you restrict yourself to a teaspoonful on a slice of bread, this shouldn't be a problem. Why not try spreading fresh fruit on your bread instead?
An occasional treat that may have a higher sugar level shouldn't be a problem provided it is eaten as part of a normal starchy meal rather than instead of a meal.