looks likes some sort of ivy i hope some one else has a better awnser

Some sort of tree seedling...maybe a Plane or a Poplar. Dig it out before it gets mohasive !!!!!

Thank you both for answering. Badfish, I think you are right about it being a Poplar! I'll dig it out tomorrow. Pity, as the leaves are lovely but a 150ft Poplar would not be 'poplar' with the other plants in my small garden or my neighbours!

You could try letting it grow and taking top off when it is as tall as you want it. This might make it bush out more but it worked for me with a self sown ash tree.

Thanks, Patfran - sadly only just got your message, so Poplar has gone. However, two self sown ash trees appeared last year and I so liked the appearance that I couldn't bear to remove them right away. I have done just what you say and will hope for the best.