OK so after some reading online, and a few videos on YouTube, I think I've found the solution...JB Weld. This is what I started with:

Held up with a shoelace so I could get to work:

It had hauled 6 of the mounting screws through the bumper, I rescrewed it to the bumper:

Then plaster inside the crack with JB Kwik-Weld and held it in place by hand for 10mins while it set, also smeared some weld on the inside and outside of the crack:

From a distance you'd never even notice it:
Tomorrow (hopefully) I'll sand down the JB Weld and then I'll have to paint the lip, I'll update with pics