The Dorset coast is warmer than Yorkshire, which is where I am, but no I wouldn't do so yet up here.

You need reliable overnight temps of 10 degrees or more. Have a look at what yours are on the south coast.

Has anyone risked putting tomato plants out yet, please?

Wet and grey 9 degrees!

The rest of the quiche with baby pots and creamed leeks again.

I find they don't do well just sown into compost & put on a heated tray. I don't think the actual compost gets hot enough for them done like that.
I put them on a big wad of folded kitchen roll, very damp, slide it into a freezer bag, fasten the top of the bag to keep moisture in & then lay it directly on my heated tray.
They make roots in just 3 or 4 days. When the root is about an inch long I transfer to compost & they grow away well.
Hope that helps.