The P21 will only reflect what Revenue already know from data supplied by employer P60, union subs etc. If you have any other items such as Medical expenses etc. you need ot tell them about those so a Form 12 would hten be helpful in capturing all income and claims.
Sending in a Form 12 will result in a P21 being issued so same thing. If you have no other income just ring and request one. If you tell them it's urgently required for a grant they will sometimes do it on the spot. Also make sure you are claiming all relevant tax credits, eg schedule E expenses, medical expenses, service charges etc.