@Simonmillett 

Yes, you can add more units.

As you have Deco M5 2-pack, it makes sense to add more M5s (and not different Deco model). Deco M5 are sold as single units, 2-pack sets and 3-pack sets. It does not matter if you buy single unit or a set, you can add all of them to your Deco mesh as Satellite Deco. See How to add an additional unit to your Deco network