If anything on Sourceforge asked for payment (other than giving a link for voluntary donations) I'd be surprised. Everything on Sourceforge should be open source.Thanks, guys.
I did a lot of hunting around, but anyithing I downloaded turned out to put "Buy this product and send us your bank details" (or something like that) across the video.
In the end I downloaded a convertor and used windows movie maker, which has a very basic increase brightness option, but it did the job.