James, although you have an excellent deposit (25%) many lenders may baulk at your varied income as they won't like the fact that you are applying for a £150k mortgage yet some months may only earn £1,000!

My advice to you would be to go and speak to your own bank (where your income is paid in). They have a direct history of you as a client and payments into your current account. From that information they should be happy to offer you a mortgage in principle based on firm and positive financial information through your history with them. 'If' you take out a fixed rate or variable rate deal with them which they are happy to offer you and you are happy that you can afford, then there will be no problem with you making additional or increased payments into the account in those better months where you are particularly flush. With your varied income the lender may advise on a fixed rate deal for you to start off with though.

Good luck.