A strong formulation of glyphosate is probably best - in the UK they're usually called brushwood killer or something similar.
Sorry I wasn't clear. I meant a ready-to-use brushwood killer formulation designed for woody growth, not mixing up the standard stuff at a different dilution.
The gel formulations are good for precise application without risk of spray drift, and I would only use it on growth coming from roots that can't be dug out (eg the ivy that was coming from under my garage - whoever laid the concrete base hadn't got rid of all the roots first).
If Niknak 2 is not in the UK, they'll need to find out what is available locally in the way of a systemic weedkiller that gets taken down into the roots. If no systemic weedkillers are available, then it would be best just to keep cutting/pulling the new growth after digging out every root that can be reached. A weedkiller that just kills the top growth won't do any better.
You're very welcome! Good luck
Just another point on gel weedkiller (sorry if it's obvious) - I find it best to wear rubber gloves and support the leaf on one hand while rubbing on the gel.
I am in the UK!Thank you all so much for your advice & knowledge - it’s very appreciated to a novice gardener