Hi, I’ve moved into an old house with high gloss walls which I want to over paint with Matt emulsion

Do you recommend a light sand then bonding primer? Or just a sand, or just primer?

If so what primer? I want to avoid sanding if I can

I would give the walls a really good clean with sugarsoap then a clean with warm water & then paint over with emulsion. That should do the trick.

Shellac based primers such as smith and Rodger blockade (sold in dulux centres) is good as a primer for switching between paint types. Basically shellac will bond well to any existing paint and any top cost will bond well to it. Bit smelly (it’s meths/spirit based rather than oil based) during application but it dries very very fast (you can sometimes feel the brittles dragging in setting paint) and the smell doesn’t linger.

Holy crap, glossed walls, takes me back.
Definitely wash, don’t underestimate the importance of this bit. Sugarsoap is good , HG liquid sander is better. The shellac based primer is very good but as stated above is very smelly. Dulux supergrip or Johnstones TRADE Ultra primer are less smelly and much easier to use.
You MIGHT get away with a just a good clean, but there’s no way I would risk it.