This issue crops up with monotonous regularity, it's just a process of elimination, first off nuke everything, and this will (usually) cause howls of indignation, fill a bathtub with a strong bleach solution and soak everything, I mean absolutely everything, bottling wand, tubing, bottles (don't forget the caps or capper) taps (pull them apart first) hydrometer, etc, then spray your fermentation chamber with it, also walls, work benches, everything! Do you have a small spare room or perhaps laundry that you can use for this exercise to create a clean room?
Bleach? Yeh, I know, but it's cheap, effective, and safe! Unlike peracetic acid/caustic soda (thank you Mark, it's good stuff but jeez you need to be careful with it)
Unlike the "sans" bleach kills pretty much everything.
Now rinse everything well with tap water to get rid of any bleach odour, then again with a starsan/stellasan whatever solution.
Get a FWK, and spray the container with the bleach solution, get 2 packs of Nottingham from a different source to what you normally use (
Cheap Yeast Store) spray the packets before opening them, spray the scissors you use to open them, with bleach solution not "sans"
Still in your clean room fill the fermenter, add the yeast (2 x 11gm packs for 23L @20 deg's, do not rehydrate) seal it using alcohol in the airlock (check this regularly as it evaporates quite quickly) and transfer it to your fermentation chamber.
Now the fun bit, after 2 weeks have a taste, if you still have an issue then I'm out of ideas, but if it's all good then you now have a baseline and can start to go back to your old ways, one at a time, and when the problem rears its ugly head again you'll know exactly what's causing it.
