Looks like it could be sun damage, but in a greenhouse the sun isn't as intense because uv A (I believe) is blocked.

Maybe put the seedling under taller plants in the greenhouse?

Good luck!

Since it's only in that spot it could also just be where the surface of the leaves was scratched / bothered by insects / possibly burned. But most importantly, it's not on the newest leaves or on any others so I honestly wouldn't worry about it at all. When the new leaves coming out have issues then I would take action, with this it looks like whatever it was has corrected itself. Cute seedling!

Thank you both for the advice, I've shifted it to different spot. Bit funny that it's the sun that's bothering it as we live in the highlands of Scotland so I thought it would suffer from the sheer lack of it(although we have been having a bizarre heatwave).Thanks again!

I would also use a magnifying glass and check for spider mites..

Just a thought.. Good luck!!

I have a beautiful vision of the highlands.. Lucky!!

Laura

Thanks for the advice Laura! It may be beautiful up here but it can be bloody cold they'll probably be on the heat mat come Autumn/Winter(if I can keep them going that is).I'll double check for spider mites as you said as we had a major problem with them on our other plants.