Shortly after fitting it, a few months back, some red "stains" appeared on the inside of the tube. They're only along around a 2 foot section in the middle of the tube and don't seem to be growing at all; they're much the same now as when I first noticed them.
I've been meaning to try cleaning it (which will have to be by disconnecting the tube, attaching a small nut or similar to a length of string, dropping/jiggling that through the tube, and then using the string to pull through a bit of j-cloth or similar....) but haven't got round to it yet, and in the meanwhile was wondering if anyone might know what it could be.....
![Image](https://i.imgur.com/u4Q374D.jpg)