Leaf size?
Leaf shape?
Some possibilities here:
https://www.rhs.org.uk/advice/common-weeds
If you're still in doubt and can take a photo of the intruder, upload it to here:
https://www.rhs.org.uk/advice/common-weeds
(Click the green button.
Navigate to the location of the file and double-click on it.
When you see a thumbnail of your image, click in the 'Direct link' field to highlight it.
Right-click and select 'Copy'
Come to this thread, right-click in the answer box and select 'Paste'.
Easy peasy!)