A First, you need to determine whether you need a septic tank or a mini treatment plant (biofi lter). This depends on your site conditions, especially the soakage potential of your subsoil. You should be able to find out from your local authority whether there are any particular problems in your area with non-mains drainage. Ask your Building Control inspector what systems have been installed locally in recent years as this will give you an idea of what will be acceptable. If a septic tank is OK there’s not much between any of the tanks on the market. Treatment plants are another matter, and besides the basic price you must evaluate the costs of installation, running the plant and annual service. The best answer is to research your options, obtaining a full breakdown of costs from the suppliers you are considering. Bear in mind that you will need a Consent to Discharge from the Environment Agency in order to discharge filtered waste to a watercourse or ditch.
For rainwater recycling, unless you are considering above-ground storage, any below-ground tanks should be installed as part of the groundworks package to avoid later disruption and additional costs.