We've decided to leave them alone and let them flourish where they are.
It's very temping to take them out as invaders but they aren't doing any harm or affecting the other plants around them. We've been harvesting the leaves and using them in salads, one of our gardening group members said they made a lovely spinach and potato curry.
I've noticed most other allotmenteers on our site haven't started planting out yet. It's REALLY tempting to get going. However,when we looked at the frost forecast for last week there was an 85% chance of ground frost on Tuesday night.