Morning notveryhappy,
Why dont you just wait until you recieve your P60 form and then ring them with the finalised amounts?...........dont they give you a certain amount of time to call them for updates?.............and yes, they can check your earnings with the tax office to confirm amounts are correct if need be.
If you've given them almost what you think the earnings were, i would'nt worry to much about it and wait to recieve the amounts you'll recieve, in the post.
If you recieve your P60's in the meantime, and a considerable amount is evident from your figures you gave them, you could just call them and inform them of the correct amount. They would send you out an amendment of the yearly allowance to you.
When they finalise your claim, you'll be paid, i would imagine, on a monthly basis, the amount that will be stated on the document they'll send you with your allowances.