ill chicken

If she is no better in the morning I would take her to the vets.  Sorry I cant give any better advice.  When they are huddled up and not eating it means theres something not quite right.  Its good that shes still drinking though.

There are a lot of things this could be - poisoning, kidney failure, an infection of some type,  I would suggest a veterinatry consultation as soon as possible.

P.S. If UK Marmite has as much salt as NZ marmite* it might not be the best thing to feed her.  High salt can put a lot of strain on her system.  If you want to put a little boost in the water then you might try a tiny bit of molasses, some soluble multi-vitamins, or an electrolyte mix.  Any mix designed to rehydrate humans will also be good for chickens.

*NZ Marmite contains an incredible 3400mg of sodium per 100g!