Can anyone tell me what this is?? Is it a weed?

It looks like an evening primrose. It can be a weed if you want it to be one.

Well I thought it was Penstomon, a red one... at least that's where I thought I planted it last year....but this year this is whats come up in its place :/
I'm happy with whatever it is, it's still pretty :/

Yes they are pretty. They can spread a bit, but it's not much of a problem..

I had Evening Primrose in a previous house. It grew into a very large bush Suzy, like Hywel says they spread a bit! Doesn't it self seed easily as well Hywel?

Yes it self seeds, but you can transplant them to wherever you want them to be ... or get rid of them if there are too many.