Woofgang
The 'custom and practice' thing referred to above is more correctly known as an 'implied term'. It basically says that if one has enjoyed the benefit of some aspect of one's employment for a length of time, it becomes an implied term and can be treated as part of one's contractual terms. For example if employees have been given holiday on Christmas Eve without it being part of holiday entitlement and nothing was ever said or written on confirm otherwise, it could be argued that the CE holiday has become an implied term.
May1day
Whilst I don't wish to discourage you, if you have something from your employer that confirms your contractual hours as x per week (and you should have, or the employer has fouled up), I don't see how whether your role is job share or part-time working changes anything in law. But do take the grievance process through, now that you have started it - a senior manager reviewing the situation under a the grievance process may be willing to accept your proposals.