I've done a ridiculous amount of research, it's taken me since April to diagnose the problem, the upside is that I know a lot more than when I started !!!
From sowing the seed in the greenhouse it all goes well, but the minute I put the seedlings (and much older plants) in the beds they quickly start to struggle. Some die within days, but mostly they just hang on and grow in what I can only describe as a 'Bonsai'd' state, I've got teeny, tiny leaves on rocket, carrots, parsnips etc etc and there's often a bluish tinge to the foliage. The soil's good, but I think that might be the problem, the beds are new and this is their first season in use, they've been filled with Rowlawn fruit and veg soil and I wonder if this is why they're outside rather than inside where they're usually found....if you can understand that logic !
I've done a lot of poking about and there is no sign of cutworms, vine weevil grubs, or any other soil based nasty, but just in case I treated with nematodes and have made repeated applications as per instruction. What I have found is very small white, wrigglers usually around 12cm or so down, under a magnifying glass they look like symphilids.
By trawling the internet I've realised that it's not something you find outdoors, especially in Manchester where I'm gardening, but I can't think of anything else it could possibly be. I'm also discovering there doesn't seem to be very much advice available, unless you're growing cannabis hydroponically.....which I'm not !!
I'd be delighted to find out it's something else, something far more straight forward, any suggestions ??
Thanks for taking the time Berghill.
PS. Another clue was not having to do any weeding, I should have known it was too good to be true !!