How can I repair this punctured pipe in my garden?
Thanks to Tyson's comments above and some reading online I figured out how to repair the pipe and even better...no leaks!

Turns out the pipe is a 20 mm MDPE pipe so it could be repaired by putting a joint in the pipe. This was done by cutting the pipe and trimming off the pierced section, then putting pipe inserts into either end of the pipe. It could then be joined using a straight pipe coupling.

The pipe inserts are required to stop the pipe getting squeezed/crushed when pushing into and tightening the coupling.

Photos of the inserts and finished fix: