Can anyone hazard a guess as to what this is

Could it be lettuce.

Looks like Rocket, I would taste a tiny piece.

Thank you - it could actually be lettuce. Definitely not rocket, as I didn't have any rocket seeds, but I did have lettuce seeds.

Will investigate further - thank you

Did you have any seeds of any of the more alternative salad leaves - lambs lettuce, for example?

Did you plant any cinneraria? The kind with silvery leaves? They look a bit like this when the come up first.

Many of the leaves look like Arugula but maybe a bit rounded. Not always easy to tell from a photo but that's what it looks like to me.

There are apps called WhatPlant or Plantsnap. I think they are free but not sure. You take a photo and the app identifies the plant. Pretty accurate in my experience.

I use an app called, Picture This. Focus on any leaf and it will identify any plant/shrub/tree followed by detailed advice on whatever leaf is identified.

Could it be rocket and lambs tail HurdyGurdy?

It doesn't look like lettuce or rocket to me. It seems to be downy/hairy.
I think you are going to have to wait until it's a bit bigger before you can identify it.

Arugula is rocket I believe and it still looks like that to me.

I also think it's rocket. I've some growing at the moment.

Rocket for sure..

Rocket and Lambs Lettuce. I don’t know where the Lambs Tail came from.

If not salad it could be Nemophila, Baby Blue Eyes.