Annie, I have to admit to you, I dunno it's name, however, I pull-out at least a half dozen of them every time I go a wandering around my gardens looking for weeds - and that's nearly every day. And yes, it even comes up among my salvias. Mind you, there are worse weeds, and that's not a difficult one to pull out.

Weed or not, it's colourfull.

it's in a part of the garden that I can't see unless I go out onto the street and walk to the furtherest end of the garden (did that make sense?). In other words I can't see it unless I make a real effort. My curiousity is the worst thing about me (almost!) and I'm half thinking, seeing as how I can't see it anyway!!, it let it grow and see what happens...what do you think?

Well, the only thing is it'll probably have a 'tap' root; a long downward growing root that anchors the plant. Most trees, in fact most seed grown plants other than grasses and the like develop them. The deeper the tap root gets the harder it is to pull out. The worst, you know, to pull out even when it's relatively small, is that of ochna serrata; that thing whose flowers change colour as they age. Birds eat the seeds and deposit them haphazardly anywhere and they germinate so readily.
Mind you, I like the birds but toilet-wise you just can't train them!

B