Does it have to be new, if you are willing to buy second hand, there is an office furniture shop that sells second hand office equipment including chairs on the corner of Honor Oak Park and Stondon Park, i.e. very close to where the P12 bus terminates. They always have stock, just need to find the office chair that suits you best. For ergonomic advice take a look at the Health and Safety Executive web site, will provide all the info you require. Suggest a high back chair with vertical movement, arm rests again that move vertically, seat that can be moved forward and backwards and up and down, and that has five "feet" on wheels for steadiness.