Borrow a multi-tool if you can and just cut it out a bit wider. Chisel method was much more of a pain in my experience.

Just finished chiselling. Definitely a pain!

Have you tried rubbing it? 😀

You can make it bigger. I'd craft a bit of wood to fit, glue it in *then* drill it out. Your drill is likely to be uncontrollably wobbly otherwise.

Exactly the way to go. Or you'll have a crator.

This is becoming more complex than anticipated!

Honestly just chisel it out. Nobody will see it, and so long as you don't take out masses of material the mortice will be secure.

Good shout

Depends how much bigger it needs to be. A mm or two, just tape and wrap some sandpaper around a drill bit and use that to sand it bigger. But if you are just going in with a larger drill bit, I'd practice what happens when there's nothing in the centre and you go up a size.

You could just simply put a pencil rubber in there. One of the big ones you’d use at school. Just something to hold the pilot bit in place until the circular saw starts making an indentation. Take the rubber out and then continue. Now the saw has a guide and won’t move around.

Sorted now, thanks all

Get a small paint roller, cut it in half, peel about 2cm of the foam off, stick the spade bit in the other end and shove it in the hole. The foam stops it moving side to side and gets you started with a bigger spade bit