The cost of sewage treatment plants in the UK vary greatly . As usual there is a market for everything so many people are prepared to pay as little as possible for their sewage treatment tank. The tanks at the lower end of the market usually have a low effluent output and will just about make the regulations passed by the envirnomental agency. The tank itself will also be pretty brittle and after a short time if the tank cracks it can start letting ground water in. At this instance you will need to replace the tank.
There are other plastic or fibreglass tanks that can be pretty expensive. They are better tanks and will give a better effluent quality from the tank, however if there is a lot of ground movement or if there is a high water table this can put a lot of pressure on the tanks and this an also crack. Once ground water gets into the tank it has to be replaced as the extra volume of water means the treatment plant will not be able cope and effluent will seep from the tank.
The alternative to plastic or fiberglass tanks is concrete tanks. These are a lot more robust and because they are made from concrete they will not crack or break. Whilst concrete tanks are at the top end of the prices they will last the lifetime of your house and beyond. This should be the main concern when buying a tank. The question you should ask yourself is “How long will it last ?” Do you want something that will last 10 years or something that will last 100 years.