Update

Well folks, thanks for all your advice - it's worked, and they seem
fine !

They were carefully introduced as suggested, and I kept Bella, the new
cat, separate for a short while in her own room to make her feel
'safe' and welcome, at least by us humans.

Then, she was gradually let out and acclimatised to the house and
other cats. (They were put outside so she could roam freely and find
the hiding places.)

The existing cats June and Smudge, and indeed Bella, all did a fair
bit of hissing for a day or two, but soon got over it.

Now they're all fed together, and sniff each other without hissing, so
real progress is being made.

Bella is entitled to just as many cuddles on the lap as the others.
Indeed, I've ensured they all get equal attention.

It's looking good for a 'tolerant' future, though I realise it's early
days.

Thanks again !

That's great news!