Can you identify this please?

It looks like a Silene or campion.

i agree with bulbaholic it is a Campion and from what i can see i'd say it was Silene vulgaris, 'bladder campion'. as it seems to be free of 'hairs'. I have been trying to get this to establish in my garden without success. lucky you.

Ooh - free plants!! I've found a couple more, they must have just blown in.
Can I transplant them & if so when's the best time?

More like a 'weed' Frenchbean :-)

Is it?! But I do like it...I think I'll keep it.

it is a wild flower or as i like to call them British natives, so if you like it keep it. Weeds are just plants in the wrong place.

I'd move them in the autumn and when the flower fades you will get a swollen 'bladder' that should have seed. so they will continue to seed around for you.

I am so envious as i can only get the red campion to grow in my garden

Thanks for the info Seaburngirl, I'm definitely keeping them as they are pretty. Thanks again.