Someone is bound to bite your hand off for such an offer (mucking out no riding that is, if you want to ride that is also a possibility but then you need a share which might be a tiny bit more difficult to find)! You just need to get access to horsey people near you because I imagine you don't want to drive for hours just to muck out a stable.
Try advertising in local tack shops and also just drive up to the big livery yards in the area (especially DIY yards) and asking if anyone needs help. Horsey people are usually quite friendly and happy to send you down the road to the next yard that might be able to help you. The Horse and Hound Online also has regional boards so it might be worth asking on there.
Hi I feel a bit of a Wally asking ( though fuelled a bit by a jack Daniels and coke) but is there anyone in the essex area that would like any help with mucking out etc? I'm not even looking to ride , although wouldn't turn it down. I feel I need to get out more , I used to ride regularly but our stables shut down. I'd just really love to be in a horsey environment again.