Advertising and Truth

In the UK the Advertising Standards Authority is prohibited from investigating complaints about political adverts for fear of unduly influencing democracy. That seems to me to be broadly sensible. But what happens when political parties make claims as fact, which they have no supportive evidence for? Should there be some sort of authority that encourages advertisers to use the word "probably" instead of "definitely" or "he is said to have" instead of "he did". Surely a clarification of what is true (supported by evidence) and what is speculation or supposition would actually enhance the democratic debate!