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The vast majority of weeds are either self seeding or have root runners. If this has been there for years without spreading dozens of others I don't think its a weed.
I have no idea what it is, but if nobody on here can offer useful suggestions, cut a piece of it off and take it to a garden centre to ask. If you tell them you are looking for ways to care for it they will soon find what it is, in the hope of selling you something to care for it.
Megs suggestion may be correct and would fit in with the massive growth.
I will be off tomorrow with a bit unless Meg remembers what it's called
Well before I swanned off to Spain I set the garden in order and then as a sop to my future neglect I flung Growmore about with a generous hand .
Perhaps that's the problem