It looks like 'leaf miner'. A tiny worm inside the leaf munching away. Cut away all the infected leaves & discard. Keep area clean, tidy & plants healthy. Consider using 'floating row covers' to prevent these insects from injecting your plants with eggs. It's the larva that do the damage. They look like leaf hoppers or tiny grass hoppers.

This video may help too:

https://www.youtube.com/watch?v=1AocFOIO9eY

Thank you very much for your help, I take it it's the same problem on both plants?

Looks like leaf miners on both. They're annoyingly persistent little beggars but as they only damage the leaves you need to find a balance between removing affected bits & being the main cause of damage yourself.
Using a tomato fertiliser rather than one that's high in nitrogen will help avoid too much tender, leafy growth but still aid flowering & fruiting

Thank you for that advice we will do that