Hi,
Just thought I would post on this message, as I work in HR (when not on maternity leave) and have answered these questions many times.
Accuring annual leave while on maternity leave: You should accrue annual leave as normal while on paid (ordinary) maternity leave (OML) - as you are unable to take it because you are on maternity leave, your employer should take any accrued but untaken annual leave to the end of your OML at the beginning of your maternity leave.
You will not accrue annual leave while on additional maternity leave (AML) - you are not paid while you are on AML, so you also don't receive other employment benefits, of which annual leave is one. This includes public holidays.
Carrying forward leave - you should be entitled to carry forward leave under the same policy as an employee who is not on maternity leave. Therefore, if your policy states that you may carry forward e.g. up to 5 days annual leave to the next leave year, you should be entitled to do so also. However, if you are off on maternity leave for a full year then you have to make sure you will be back within the annual leave year to have the opportunity to take the carried forward leave - post policies will state that you cannot carry it forward across more than one leave year.
E.g. your annual leave year runs 1 Jan to 31 Dec. You are on leave from 1 Nov 05 to 31 Oct 06 - you should be paid any outstanding leave for the 2005 annual leave year when you go on leave in November. If your policy allows it and you choose to do so, you could carry forward 5 days' annual leave into the 2006 leave year. This would then not be paid when you go on mat leave, as you have chosen to carry it forward. However, you would then have to use this between 1 Nov 06 and 31 Dec 06, or you would lose it, as you can't carry it into the subsequent 2007 leave year. It is also right that you have to return from maternity leave before you can take annual leave, as you cannot start receiving/using benefits again until you have resumed paid employment.
Bottom line is that you have to be treated same as any other employee who is not on maternity leave, so if your policy allows employees to carry forward leave from one year to next, all employees must be allowed to do so, regardless of whether on maternity leave or not. Similarly, the same restrictions as to how long you can carry leave forward for should apply to all employees.
Hope this is not too long-winded a response and is helpful!