Hi Catrose and welcome to GoY. Potatoes, like most vegetables, do best if grown in an open sunny position. Your potato tops have grown as tall as they have because they were seeking light. The only way you'll discover if you've got a good crop is when you check on them which you can do by putting your hand down into the soil and feeling around. Our first early potatoes have been in for about 12 weeks now and are only the size of marbles. We're putting that down to the cold, wet weather and the lack of sunlight. Hopefully they will grow larger.

Silly question, but, you are topping the compost up in the bag? In a shady area the green growth will be drawn towards the light. On my allotment last year, had I tied them up, my potato tops would have been about 3 - 4 feet high. I cannot say I am totally convinced that bag growing is worth the effort and cost.

Hum, unfortunately I don't have a lot of choice about where to grow, so it prob is too shaded. Oh well! Thanks for your replies, appreciate it.