I use half a dishwasher tablet/ full one if really dirty, hemp, oats etc. Drop half tablet in the flask, pour boiling water on it and leave overnight. If you are feeling a bit squeamish next morning don't look at what's floating on the top.I looked into my flask yesterday and decided it really did need a clean. Tried a bottle cleaning brush and cillit bang but was hard work and only a partial success.
Any other ideas that really work?