Our local Family History Society is the Buckinghamshire FHS. It encourages the study of family history in Buckinghamshire, catering both for those with Bucks ancestry (wherever they may be living) and for those living locally whose ancestors may have lived elsewhere. Much information is available online (some chargeable) and the link to their pages relating specifically to the parish of St James, Great Horwood is here.
GenUKI is a reference library of genealogical information relevant to the UK and Ireland. It includes a page on Great Horwood with extensive historical, census and photographic details.