I'd be digging each bit out and making sure I got the roots too, which is clearly what wasn't done initially. You have a problem with the weedkiller situation - this is a woody plant and as such responds better to a brushwood killer than it will to other killers which deal very well with soft growth plants. So SBK is the best, but you would need to drill into the roots and apply the SBK to the tunnels you've created. It cannot be watered onto the soil, because it will kill everything and prevent your growing anything else for a good six months.

Interesting, we have Cotoneaster Horizontalis in the garden but it doesn't spread - why is it called horizontalis I wonder when it is actually vertical?

No its not vertical, MG - in an open area, it spreads out in layers horizontally, often rooting at the tips as it goes. Only vertical if grown against a fence or wall.

Hum... okay

lol, you don't sound convinced, MG!

Just been outside to look and yes it is against the wall of the house, I thought it was actually standing free of it.