I am sure you will get lots of comprehensive answers, but just briefly, it depends on the local authority you are working under. Universally though, the school will be expected to fund the first £6000 of support and then the LA will top up to the total amount on the plan. So you need to know if your £12,700 includes the delegated £6000. Some LAs have gone back to hours which is worse really. You may not want to put all the funding to LSA time anyway? so you need to have a good look at expected outcomes and provision required and you might find there is a good solution to how many actual hours you decide to give.