I don't know whether you have regular performance appraisals in your company, but I would sit down and think of all you have achieved in the past 12 months and list any spectacular performances which have benefitted the business / improved sales / productivity, etc.
I think if you can go armed with some positive ammunition you will give yourself a better chance of showing you have earned the right to more money. (Unfortunately getting yourself a bigger mortgage is irrelevant to an employer, but an employee who has brought genuine benefits to the business is a different matter). Once you've got these arguments clear in your head, ask your boss for a specific time for a meeting, rather than catching her on the hop, and then go in a state your case. I think a face-to-face conversation is better than a letter and if you're nervous about doing this, rehearse your request in your mind beforehand.