It isn't hogweed, can't i.d. for you sorry. Hogweed looks like cow parsley on steroids.

It's the white flowered form of Red Valerian (Centranthus ruber 'Alba')

Thank you for your responses. That puts my mind at ease. I will now research Centranthus ruber 'Alba' Thank You

I think valerian is good for butterflies. It grows in every nook it can around here!

People seem to have mixed opinions, it can take over but can also look pretty.

i cant get it to take at all in my garden. neighbours have it either side but not for me :o(

definately valerian, butterflies and moths like this , like bryony says it can grow anywhere,
I used to have this on a bank in my front garden where i used to live we had the red ones and a pink one as well

Wow thanks everyone , you are all great. Now I know what it is I wont dig it out in fear of it being Hog Weed .
Seaburngirl Im not a great gardener (novice) and am surprised that this has taken so well hence why I thought it was a weed.
Thank you all

As soon as the flowers fade, remove them or you may reconsider your last statement Carie lol. This promotes a second flush of flowers too.