Cutting slot for mortice lock

[andykent]

Replacing 6" mortice lock but hole slightly too small for new one.
Does the panel recommend drilling, chiselling or both, and if chiselling what should I be Googling for to get one long enough? Basic chisels are too short and fancy longer ones seem to be at antique prices.
Sorry if this sounds a bit 'duh' but it's all proving more tedious than I thought.

 

[ladylola]

Chiselling is the way to go . As you say the longer chisels are expensive and probably not worth the expense if you aren't going to need one again. Some time ago I needed a long chisel and not having one at the time sharpened up a metal gutter bracket. Obviously it didn't stay sharp anywhere near as long but as I was cutting out a slot in an old oak beam it did the job surprisingly well.
Edit
Just rereading your post , if the hole is too shallow then a drill will be needed as well.

 

[foxhole]

Chisel is all that's needed, you can deepen the hole with chisel as well as widen, just takes a little longer.
You can also just use a long bit and ream the hole if you are careful.

 

[ladylola]

Ideal tool is a swan neck chisel , says I smugly knowing there's one in the workshop. Old one that I picked up at a market for around £15 . Not something that gets carried around that much but useful for when I know I'm going to need one.