Coltsfoot, help

I've finaly indentified that I have a few (make that loads of!) Clotfoots' coming up on my plot.

I glyphosated a few weeks ago now and everything yellowed and died except the Coltsfoot, it seems that the weedkiller didn't work on it.

I've been on digging over the treated area, over the weekend and noticed the 'tubers' go down a long long way.

So I would like to know, does the Coltsfoots' leaves have to be 'bashed/bruised' like Marestail if I have to re-treat anywhere, or dig down to almost hell (up to a Metre according to the web) as I'm digging??

And, what are the chances of it coming back once I've got as much tuber out as possible, does this weaken it?

They were visible, but only the size of a penny really, this is how I can identify it now, but not then, they were just poking through.

Do you think maybe that wasn't enough surface area for glyphosate to get a hold and do it's job?