Each of the big plants (which must be about 5 or 6 years old now - I can't really remember), had several thick woody stems which were bare down below but bushy up above, like this:
I decided to remove most of the old stems. So, out with the long-handled Pruners... Five minutes later, this is what they looked like:
I left in place all the young pliable stems, but I removed most of the woody ones.
Two of the plants didn't have any young stems coming out of the base of the plant; only bushy growth at the top of some very thick old wood, so I removed most of the old stems, leaving one or two behind. If new basal growth appears, I will remove the last of the woody stems next year.
I also trimmed the bushy growth at the top of these old branches. I expect more new shoots will soon appear.
In order not to put the plants under too much stress I am being very careful to keep their compost moist, and I have placed their pots in pot-saucers to minimise water loss.
I'm not sure whether this is the "approved" method of pruning Blueberries, but it's what feels right to me. I'll let you know how things go.