Chris,
All of your rights, bar the right to remuneration, must be maintained throughout the duration of your maternity leave. Under European law, "remuneration" is limited to the payment of wages or salary.
In others words, you will not have an entitlement to a bonus payment in relation to your period of absence on maternity leave, if the bonus payment is in the nature of wages - i.e. it is a monetary amount based on personal performance / performance targets.
If that is the case, and your bonus is based on meeting performance targets - your employer may reduce your total bonus on a pro-rata basis. It should be noted that your period of COMPULSORY maternity leave (i.e. the two-week period around your confinement that you are not legally permitted to work) cannot be taken into account in any pro-rata reduction.
e.g. If you are absent for 4 months on maternity leave, your entitlement to an annual performance bonus may be reduced by one third - but your employer must also give a credit for the period of compulsory materntiy leave. In effect your entitlement will be reduced by 3months 2 weeks.
Alternatively, if the bonus payment is not linked to performance targets, you will be entitled to 100% of the bonus as it is not in that nature of payment of wages or salary.
Thus, if your company offers a bonus for people who stay for a certain amount of time, your qualifying length of service for such a bonnus cannot be reduced by reference to any periods of absence due to maternity leave.