How often should I submit my expenses?

You should send your expense claims to us weekly. Expenses are offset against your earnings, and can only be offset as far as National Minimum Wage, therefore, if you have a large amount outstanding when you leave we will have nothing to offset them against, and we will be unable to offset those remaining.
You are not really losing out because we will have offset as much as we are allowed to by HMRC rules, however, if you can keep on top of sending your claim forms in on time, this will mean we can offset as much as we can every week for you.
If in any week the amount of expenses exceeds the amount we are able to offset taking into account NMW at that time, we will offset as much as we can and carry the remaining claim forward to the following week.