I'm not an engineer, and I'm a novice with defenders.

My intention with this post is therefore not to show off, it is purely to help others in future with the same perceived problem.

I honestly thought I was going to have to grind down a 13mm spanner to be narrow enough to undo the nuts on the threaded section of the master cylinder that attaches it to the top of the clutch pedal.

As the photo shows I managed to use standard tooling.

13mm nipple spanner and an extension bar on a 13mm socket driven by ratchet.

Hopefully this helps others with their first time.

Let me know if the picture hasn't uploaded as this is first time I've tried an image.