Thanks for the pictures Fishkies, I have to be honest and say I don't know if they look like that or not, they're just too tiny to see properly.
I shall have to wait and see then! They're so tiny they're nearly impossible to see, at first I thought it was some kind of bug and tried to suck it up with the gravel vac! When it wouldn't go up the tube I had a closer look and realised it was a fry! So hubby and I had a search around the tank, moved a few things and found another one.
It's definately not Molly fry as molly fry look like mini-mollies from the time they're born and are much bigger.
But as I've never had guppy or platy fry I have no idea what they look like in real life.
These guys (from the side) look like and eyeball with a transparent smudge around them (the body). If they're still alive tonight I'll try to vac' them up and put them in the sieve. There's chances of survival in the "mum's & fry" tank is not good without a bit of intervention. If I manage to get them out I'll take a picture and post it.