Many of the mortice latches on my internal doors are not as smooth as
I'd like. I think they're all about 20 years old, so I'm planning to
replace them all at once rather than just replacing the bad ones.

I know there's a a nasty failure mode with these latches where the
die-cast insides can fail leaving you locked in or out, and I'm keen
to avoid cheap-and-nasty brands that might do this if possible.

There are a large range of prices at screwfix - from 99p each to over
5 pounds. Is price any indication of quality with these sorts of
items? I'm wondering whether it's best to buy a branded one like
Smith and Locke (which maybe just a B&Q / Screwfix brand anyway) or
just go with the unbranded cheap ones and maybe replace them every few
years.