What are sugar-free sweets actually made from?
It appears that he main ingredients of sugar free Polo Mints are:


Sorbitol - a non-sugar (technically a sugar alcohol) sweetener with less calories per gram than sucrose, about 2.6 kilo-calories per gram compared to sugar's 3.9.  E420 in Europe.
Magnesium stearate - this appears to not be metabolizable.  E470b in Europe.
Mint oils


I did a quick check of UK labeling laws, and it appears all ingredients must be listed.

So the answer is:  the sugar substitute itself, sorbitol provides some of the bulk of the candy, with most of the rest being the magnesium stearate.