With hindsight and more experience I wouldn't now call Mayday in this situation but after speaking with ATC at Cambridge and the military they were OK with how I'd handled it.
Sounds like a good call given your circumstances at the time. I'd have been !!!!!ting myself if that happened on my first flight post ppl. What constitutes a "mayday"' situation is subjective and at least calling mayday prompted ATC to sit up and take notice of a potentially dangerous situation evolving, and it obviously ended in a safe resolution.
I imagine there was some paperwork to do afterwards?