I can't give you chapter and verse @SausyFC but it's more prevalent between 6pm and 6am GMT.

There are two that work that have the same intro as the one that fails and I don't see them between those times but do at other times, the promo for the new Train your Dragon movie and the Bank switching ad...

NB These ads do not have any ident or logo as to the ad provider, in the way that others (eg AdChoices) do.