Help identify a Pest please

[Biscuit]

I've got some young spinach beet in an indoor container. The leaves are being nibbled at by some very small grub or worm things. They are about 4mm long and 1mm wide (or less). They look translucent, worm-like. Don't move much. Very hard to see without searching. There have also been some very small flies hovering about recently which I guess is related.

Any ideas? I don't fancy it getting worse.

[Annpan]

I have never seen the grubs, but I frequently used to have problems with tiny flies around my indoor plants.
I found info online about the soil being wet is what causes them, so try to water the plant in the tray underneath (for it to soak up) or use some kind of funnel in the soil, to take the water down below the surface.

HTH