Go to amazon.co.uk and buy 12 month supply of nbs sprang septic tank treatment. Aroundn£20 with free shipping to europe. Best i've used in 10 years and bleach resistent too. Drop 1 pouch a month in loo, give it ten minutes then flush away. Read reviews on Amazon, it is ws good as they say. But, be punctual with monthly treatments!!  Thassall. 8-)