I just did a "dummy" booking to check, as I've still got six tickets left from 2019, and the supplement is £11 or £31 depending on train times; I think the £66 they quote in the T&Cs is for a flexiplus frequent traveller, not for the ordinary ones. But good news that they've extended to the end of the year, am coming up to a busy time at work, so was going to have trouble fitting in three trips before the end of June.