Looking at just the picture, I would agree likely water coming down the pipe during off season caused the rot. I don’t think you could have caused it to burn thru in that relatively short time ( yes I am that old :)) I would put a wire brush of grinder to it to determine how bad it is. If not...
I would suggest you definitely contact AB Lumber in Pembroke and Center Hill Barns in Chichester.
I went with Center Hill with mine after doing the research a few years ago and was very
happy with their work
One more reply, I didn't have the money on mind so I cheated out and then had to retro fit.....Yup spend it now, you will be glad you did on the first hot summer day