In reply to Sean Kelly:
I left one half full with Gatorade for about 3-6 months once in a warm kitchen. Truly nasty. Managed to get it all out with brushing and shaking with bicarb. Shaking with washing up liquid just foams and stops the water hitting the sides as hard when you shake.
just to add to the baby bottle brush thing. Superdrug do one now that is foam at the tip and has an extendible handle so it is ideal for getting into corners.
http://www.superdrug.com/Munchkin/Munchkin-Bottle-And-Teat-Brush/p/999983#....
Another tip is after you have let the bottle drain you will find that beads of water will stay in a Sigg even when upside down for ages as much as you shake it. I nab the missus hair dryer and if you alternate between blowing hot air over the opening and heating the outside you can get it bone dry for storing with the lid on in a minute or so.
If you have gunk stuck inside it rather than any lining peeling it then warming and cooling probably helps to descale it too. If you think it is a lining (I think they changed material of the liners 10 years back or so)coming off then sieve the water as you pour it out to inspect. Chuck it if it is obviously flakes of such.