What's the current best compost for seed sowing ?

Well, I opened the bag and it's quite fibrous stuff to look at (sure I heard the sound of bongo drums when I did), was rather wet, and has some lumpy-ish bits in which can be teased out ok. Planted some larger seeds but used a bit of John Innes No 2 soil on the top, sowed the seeds, then covered it with a bit more soil.

This is Gardeners' World's answer to the original question https://www.gardenersworld.com/product-guides/growing/best-peat-free-composts/ Some very expensive composts, and I am not sure why they should be.

Also came across this which I was not aware of. From the Which site: Store your compost in a cool, dry place, and try to use it up within a few weeks. Which? Gardening trials have shown that peat-free composts often start to rot down after a few months and the fertiliser in them breaks down, which might harm your plants. Store it in a shed or garage, but if you don’t have these, keep it in a shady spot, covered with a tarpaulin.

edit to fix link

I sometimes wonder what was done 50 years ago

Funnily enough putting Hyacinths into 'bulb fibre' about a month before Christmas, with my Dad, is one of my earliest gardening memories. 

This year I pushed the boat out and brought a few bags of expensive organic Melcourt SylvaGrow peat free multipurpose. Have to say I was disappointed - the compost was quite heavy and grew mushrooms. Despite being mixed with 50% grit for growing some aeonium cuttings and potting on come summer it was clear to see some of the plants were unhappy and I repotted with John Innes 2 mix. Come summer it dried out very quickly for potted tomatoes.

I've been peat free for a more years than I can remember, 2021 I used Westlands New Horizon all veg peat free and was disappointed, this year its been a slightly better mix.

I wish the composting industry would simplify things - John Innes is dead without peat. The RHS should stipulate idea mixes for seeds, potting, mature plants and suppliers should stick to this, with pure green waste/organic matter labelled as soil improver for clarity to those new to gardening.

2021 we ordered 3 tones of recycled green waste - it was full of plastic and glass and still warm 4 months later. Never again! I was hoping to reduce plastic bag consumption!

This year I pushed the boat out instead of finely sieving whatever multi purpose compost I had lying around and bought some Levingtons John Innes seed peat free compost. I have to say I was very pleased with it and most of my seedlings went on to grow into nice healthy plants, especially my tomatoes. I'll be buying it again for next season as I found I was able to reuse it a couple of times after the first sowings by simply sieving it and sowing new seeds. 

I'm going to try a little experiment.

For seed-sowing in modules, pots, etc., all that really needs to happen is for seeds to finish germinating and produce enough robust roots and leaves to be able to transplant, moving on to the next step, whether that is a slightly larger pot, or planting out in the ground.

I'm going to chit some more spinach seeds, then put those into small flats with nothing but river sand.  the flats will be watered with tap water mostly, with an occasional weak feed of diluted fertilizer. Just watered enough to stay hydrated. I'll see how they do.

Hypothesis I am testing: Seeds should be able to get started in just about anything they can sink roots into, growing enough to get them to the next stage.  If it works, maybe the best seed starting compost is not necessary?  I could be completely wrong, we'll see.  Watch this space.

Following up on my experiment.

I started chitting the spinach seeds within a day or  two after posting. I prepared 4 small flats with
(1) peat-based Miracle Gro seed starting compost (Mostly peat, some perlite, a little wetting agent),
(2) Fine river sand, collected from a sand bar in our local Oconee River, brownish, micaceous.
(3) Coarse white play sand from the hardware store (just happened to have some lying around)
(4) On the right, compost produced from green waste and biosolids, processed by our city.  What I had on hand has been aging behind my garden in a pile for about 6 months.  Lots of millipedes and other critters in there, I made no attempt to get the bugs out.  Some bits of wood in there, but most of it is fine, loose, black organic matter.

In flats 1, 2, and 4, I put the chitted seeds in the flats, about the same number in each.  Number 3 (play sand) got the short straw; whatever seeds I had left, chitted or not, after pulling out the strongest chitted seeds for the other flats (so, not exactly a fair comparison).  Here's what they look like after a couple weeks, 1 through 4, right to left, in the attached picture:

My assessment is that the city compost was slightly better than the peat compost.  The fine river sand performed well enough, it might have performed better if I followed through on the intent to water with a little weak fertilizer, it did a reasonably good job retaining moisture and the seedlings grew.  The coarse play sand was not great, but did manage to produce 18 healthy seedlings from not-so-great seeds.  The seedlings are a bit leggy, it has been overcast most of the week.

I've moved the play sands seedlings onto modules with city compost in them. I will do the same thing with the others this weekend.