What are they?

As the seedlings develop I begin to get some inkling of what they might be. The first to germinate last autumn (lowerpicture) has produced a trifoliate second true leaf and it reminds me of one of the trefoils or clovers. The second up has further slightly serrated leaves and may be groundsel.

More seedlings now appear almost every day and the large specimen spotted recently has now pulled its long cotyledons out of the earth and might be an ash tree - rather too large for a window box!