It's Centranthus ruber - common name is Valerian, though it shares this name with a completely different plant.

Weed or not? It appears in lots of gardening books. If you like it, grow it; if not, call it a weed and haul it out. The choice is yours.

The roadsides in Cornwall are full of it at this time of year and it looks attractive, but it is a spreader / self-seeder so it's easy to end up with too much of it.

Yes it's what most people call Valerian. Some like it and some don't. It can grow upright in the right situation but I am one of those who pulls it up because it can get everywhere. But many like it and want it in their gardens.
As Beattie says, the choice is yours.

Thanks for naming it for me. I googled it and it appears to be a flower and people actually buy it!? I think I will pull it out where it is straggly and untidy and keep a few to try and keep tidy.

It's amazing sometimes when you go to the GC and see plants offered for sale, and even being bought, that you pull up on sight. I've had to button my lip a good few times in the queue to pay - thinking "DON'T BUY IT!!" :-(