Thanks very much guys. The purpose of my original question was basically to see if I could skip the jigsaw step and, using the sheet with the hole already cut out as the template, cut & trim in the one operation with the router.
Y'all didn't seem to like that idea too much so I took your collective advice and again did the jigsaw thing first. Job is now finished and still looking pretty good.
Certainly glad I found you guys – lots of good information provided. Just hope I don't get ahead of myself as this router gadget is giving me all sorts of ideas that are probably well beyond my capabilities at the moment :-(
Anyway, I've still only use the Ryobi cordless trim router, so I guess it might be wise to find a project where I can get a bit of practice with the plunge routers. I was thinking that a DIY router table might be a very handy addition to my slowly growing workshop. I have been looking through YouTube, but so many to choose from I'm having trouble making a decision.
Just a few comments...
Stick486: Thanks, I really appreciate the list of PDFs. I've printed them all and will work my way through them.
Quillman: I would have only been cutting 16mm MDF sheet, not 35mm. The benchtop now consists of 2 x 16mm sheets, one atop the other, but your warning is noted. Yes, my first encounter with routing MDF alerted me to the dust dangers. I have a good multi-filter Protector Respirator.
Everend: I must admit I didn't even think about using scrap pieces instead of a full sheet. Anyway, it's measured cut and done now.
Again, thanks to everyone for your help.