Desiree is a main crop potato so I wouldn't expect them to be ready now. I would wait (if you can) until, at least the end of September or until the haulms die down. First earlies would be a better choice if you are going to grow in bags. They will as the name suggests harvest early and have shorter growing period. New potatoes with butter and mint are greatly appreciated when they are still scarce and expensive in the shops.

Thanks Jimmytheone

I have pulled up another two and they are still small. I will leave them in as you say. I need to space my pots out a bit better next year as I have had rather a gap between harvesting my second earlies and these. I have just put in some Nicola and they have started sprouting already.

Thanks again for this advice!