I'm sure it can and does work for some.
Some people will blindly dip their hands in their pockets everytime they see a collection tin; others will only donate to a charity that has a personal meaning to them.
Plenty are just tired of giving.
Unless you introduce some kind of 'checkout for charity' or manage to increase the site visits, I cannot see a way of increasing your donations... it's up to the individual.
This is meant to be a positive post and I admire your commitment to Mencap.
Keep at it!