I have Swift First Earlies planted in a rubble sack in the greenhouse and two more sacks, of Pentland Javelin and Home Guard in the shed. They have been covered with potting compost as the green appears and the greenhouse sack is already full, so the greenery will now develop as in your bucket Tim. Those in the shed still have a way to go, but I'm quite happy with this 'staggered' growth. It will give me new spuds for a longer period of time - don't have room for spuds in my small garden and since my mini-greenhouse is packed to the gills with seedlings, the sack will soon have to take its chances outside in a sheltered spot.
We ate the first Swift potatoes on 28th May last year, so not long to wait now ;D
Tricia