A useful “factcheck” but it doesn’t make absolutely clear the difference between a threshold based on REGISTERED voters and one based on votes actually CAST. Cameron is right in the particulars – his type of threshold based on CAST votes hasn’t been used before in referendums in the UK
The difference is very material. The first is beguilingly democratic yet encourages abstention campaigns designed to discourage people from voting because an abstainer counts as a NO vote. Neither is it at all transparent as to how it works. In Scotland in 1979, on the 60% turnout if my memory serves, Yes needed 83% of the CAST votes to scrape a win over the 40% REGISTERED threshold.
While I would reluctantly accept a threshold based on cast votes, I would be totally opposed to one based on registered votes. Such thresholds have rendered referendums a discredited device in Italy and several other European countries.
Any turnout threshold should be set at a level to avoid ridicule in a very low poll than as a democratic hurdle. And a fixed threshold like 40% would represent a variable hurdle in practice – easily met if you proposed to abolish the army as Switerzerland proposed in a referendum in the 1980s and harder as in a low interest issue like the AV referendum.