What do you think of this overpayment idea?

Just started this at the weekend, views, comments and enhancements welcome:-

Thanks to this site I budget mine and my partners income quite tightly, we have all our outgoings, annual outgoings etc. then we have money left over for social activities. Because sometimes we seem to be busy in a particular month, i.e. family visiting from London, somebody's 30th, bank holiday outings etc. we try to budget for out nights out. For example, this Saturday is a mates 30th and we are having an all-day session in the lakes. I aim to take £60 for beer + food to last 10-12 hours. Whatever money is left, i use to overpay the mortgage, so if i only spend £50, then I overpay by a tenner. I budgeted £60 anyhow so what the hell?