Hi Mick and welcome to GoY. Difficult to tell but it 'looks' like a small fungi - possibly one of the cap fungi. The compost also looks extremely wet which would encourage fungal growth. I would remove and let the compost dry out a little.

What a splendid fungus! However, it has no right to be there. I second MG's advice, let that wet compost dry out a bit and if you can get hold of some, spray all your tomato seedlings with Bordeaux mixture. (copper sulphate solution) Most vegetable seedlings of all kinds are routinely sprayed with this by commercial growers - you can tell by the pretty blue tint. (Well they do here - same in the UK?) It acts as a fungicide, mostly, I suspect to protect against damping off.

I think the commercial growers in the UK use nastier substances Gattina

Gosh, that sounds sinister! WHat do they use and why are they allowed to do it?

While holidaying here 7/8 years ago we often wondered why tomato plants appeared to be 'blue.' We now know it's copper sulphate and appears to be a necessity as every little patch of ground with veggies is the routine 'blue.'

A systemic fungicide and herbicide Gattina - though copper sulphate is not particularly a goody either! It builds up in the soil.