I don't think you be able to mend it, it will have to be replaced I'm afraid. You'll also need to find out if there is a leak somewhere so that can be fixed too. Otherwise it will just rot again afterwards. Maybe this video will help give you some ideas of the time and possible cost involved to fix it. - https://www.youtube.com/watch?v=7CghhgkKY9k
It's a pleasure Stella, glad we could help
Hello Stella, Do you know where the water or damp is coming from? First get that sorted otherwise it will keep happening. The wood will need to be striped out and replaced I'm afraid!
Thank you wasn't sure where to start x