[Tricky]

one of my chickens has just started doing watery white/yellow messes. i had a look at the poop chart and mine is nothing like any of them, it looks more like a very watered down egg, pure white (when dried) with a blob/lump of yellow. she is also very lethargic, and although drinking, is not eating.
not quite sure what the problem is or what to do. 

[SusieB]

Although it seems almost any poop can be ok, if she's not eating it is not a good sign.  My ex-batt had yellowy poop and it was a problem with her ovaries/reproductive bits.

If she is lethargic and not eating, she may have been poorly for a while, as chickens hide illness well. 

Hopefully somebody more experienced will be along soon to give you more definite advice.

[New shoot]

The watery poo could be a symptom of this hen not eating, but SusieB is right to point out that this hen may have been unwell for a while.

Assuming your hens are wormed regularly and you have done all the usual checks for sour crop, impacted crop, prolapse or other problems around the vent, the next option is something internal which you are not going to be able to help her with.  At the moment she is suffering and starving, so she needs some urgent medical attention or steps to put her out of her misery