I should have posted an update here rather than in the comments (but I wanted to test the comments out again anyway). The update process was fairly simple. I had made some changes to the code for 3.15 and I forgot about those when I upgraded to 3.20. Apart from that, it was simple as transferring all of the new files to my webserver and going to the main page and running the upgrade tool. No interaction from me required at all (once I remembered what changes I had made).
Some great work by the MT crew to produce such an easy update.
I’m finally getting round to upgrading my MT installation which I am intendning to start in about 5mins.
Point being, if you see this post in a few days, you know I’ve screwed up and I’ve not fixed it yet.
Now the instructions say it is easy… We’ll see.