Crossed with NLMum27 but....
A bursary is purely based on parental income/outgoings (ie means tested) and, therefore, has to be reassessed every year as, for example, you could be unemployed the first year and then win the lottery the second. Usually, any scholarship award is on top of the bursary so, in theory, you could get the equivalent of a "free" place (with the caveat that this usually covers tuition, not necessarily all the costs associated with school).
In practice, unless you expect your income to suddenly shoot up, then it is likely that the bursary will remain about the same level - however, there is always the chance that the bursary pot in the school will be reduced and less will be available generally, across the school (they cannot guarantee to offer the same level of bursary across every year, even if your income didn't change, things in the school might). in practice, in that sort of situation, most schools tend to try and main the bursaries they have already awarded and the effect is felt more with new entrants to the school.
Scary mum is tight that all bursaries and scholarships carry "conditions" - if a child is not performing at the expected level - or, in the case of some scholarships, contributing to the wider school life, the money can be withdrawn.