I’ve had a good read through the weekly rest thread, and I now understand how it works.
You can have two reduced in a row. Where a regular rest normally counts for the week it leads into, if you have two reduced in a row, then the first regular will have to count for the 2nd week that has a reduced. So where you normally work it forward, you then have to count it at the end of the previous week.
That might not make sense, but I get the idea now. This will continue until you can have a rest of 69 hours (bank holiday for example) then you can then start working the counting forwards again.