In answer to Frostyfingers, it is in Section H2 of the Building Regulations that septic tanks require emptying at least once every 12 months. Every septic tank manufacturer will also tell you this. They design the tanks with only 12 month sludge storage capacity before settlement in the tank is impaired.
The bacteria can never digest ALL the waste. The organic material is reduced by about 70%, leaving septic sludge at the bottom of the tank, but the soil, from washing vegatables, washing dirty clothes (gardening, rugby kit, etc.)washing the dog, etc. is inorganic rock and cannot be digested at all. Soil and the sludge builds up in the tank, if the tank isn't emptied, reducing the settlement depth, which in turn reduces settlement - a vicious circle. The result is that increasing levels of unsettled solids leave the tank and enter the soakaway, causing clogging of the soil and the end of the soakaways life. New soakaways are very expensive!
The tank does not have to be 'full' of solids before it stops working - the soakaway will be ruined long before then!
Frostyfingers is correct in that it is very important to use the right products to avoid poisioning the bacteria.
In answer to 'Millarkie', it is not soft water that is harmfull to the septic tank bacteria but the water softeners regenerate water which contains massive amounts of salt. This level of salt will kill the bacteria. It is OK to install a water softener as long as the regenerate water is drained away from the septic tank