Quote:
"The maximum number of Rollovers is four. This means that if the Jackpot is not won in five consecutive Lotto Draws, in the fifth Lotto Draw the entire Jackpot will roll down to the next Lotto Prize category where there is at least one winning Lotto Entry"
Source:
https://www.national-lottery.co.uk/games/lotto/game-procedures
So if there are no '6 number' winners the jackpot is automatically added to the amount distributed between the '5 number plus bonus ball' winners.