Swallow Box Crashers

Hello,

A few years ago I had a swallow box fitted high up on my house wall by a local swallow society (we have a lot coming to visit in the summer). For the last two years, a great tit has built its nest in the box, however because the box is designed for swallows, the hole is flush with the base, resulting in the fledgling tits falling out before they're ready to fly.

This has happened yet again this year. Does anybody have any suggestions how to either prevent the tits nesting in the first place, or from falling out? (Getting up to the box may prove tricky as it is next to tithe roof line...)