[mumofstig]

seems to settle down a lot, which doesn't seem to help seedling roots. I wondered if I can lighten the mix for the next lot of seed sowing - but with what?
Which would be best, Perlite or vermiculite?

[Auntiemogs]

"Perlite and vermiculite are both good at retaining water, but vermiculite acts more like a sponge, holding much more water than perlite and offering less aeration for the plant roots. Perlite retains water because of its large surface area with nooks and crannies available for water storage. Because it is porous it allows excess water to drain more readily than vermiculite and improves soil aeration."
I (personally) would go with perlite. 

[crh75]

I use a bit of both! But, as said above, would favour perlite for most things.

[sunshineband]

I use perlite for the reasons already offered

I find that this makes a very good mix for sowing:

One third perlite
One third reconstituted coir
One third mpc

Not much in the way of nutrients so weak feeding once first proper leaves have grown, but it really is great for germination for seeds of all sizes. (Tiny ones get a sprinkle of perlite over them when sowing if they need covering)