SnowD & Jan are both right.
This is from the Drinking Water Inspectorate site
Drinking Water from Storage Tanks
Is stored water safe to drink?
The quality of water taken from storage system in properties can be impaired unless adequately protected (see below "What if I do have a storage tank?". Modern practice is that drinking water in the home is supplied directly from mains - and not from storage.
That does not mean that stored water is harmful - but it must not be taken for granted!
Do I have a water storage tank?
In homes with unvented heating systems, both hot and cold taps deliver water direct from supply at the pressure in the water company’s main. They do not have a storage tank in the loft, and all cold tap water should be of drinking water quality, as supplied by the water company. In other houses, the kitchen cold tap supplies water directly from the mains, and in many the bathroom basin cold tap does too. Generally, water from taps direct from supply is at higher pressure than from taps supplied from storage.
If you are not sure whether a tap delivers water direct from supply, your water company or a plumber could advise you further. A quick check plumbers make is to compare stored and directly-supplied water pressures by placing his/her thumb over the tap outlet to "feel" the water pressure.
What if I do have a storage tank?
Water from storage can be safe to drink, providing the cistern:
is not too large, so that standing water is limited (25 litres (5 gallons) per home is the guideline);
is flushed to waste should the water not have been used recently, depending on circumstances such as warmth and number of days unused;
is made of approved materials (such as listed in the Water Fittings and Materials Directory, published by the Water Regulations Advisory Scheme (WRAS) www.wras.co.uk; telephone : 01495 248454. Further Information on services provided by WRAS can be found below.
has a lid which excludes light and is tightly fitting and securely fastened, so that birds, vermin, and dirt cannot get in;
has vents, including overflow, that are screened so that insects and particles cannot get in;
is insulated and in a cool place so that, ideally, the temperature does not exceed 20° C;
is cleaned, and disinfected annually as recommended. Cleaning is very important in order to remove even inorganic debris which could provide a habitat for bacteria, including legionella; and
is periodically sampled to ensure that too many bacteria are not present (depending on circumstances, between two and four times a year is normally advised).
However, water quality in storage can deteriorate, especially, when;
there is no lid on the storage tank;
water is warm;
water has been in storage for too long; and/or
the cistern is made or lined by material which is unsuitable.
To avoid the deterioration of the water quality in your storage tank, the above advice should be noted
When water arrives at your property
Generally, the building owner is responsible for all aspects of the plumbing system and its maintenance, which includes the part of the supply pipe from the boundary of the highway in which the public water main is situated. New plumbing systems, and alterations and additions to older systems, must meet the requirements of the Water Supply (Water Fittings) Regulations, 1999. Older systems must continue to meet the requirements, generally, of the water byelaws in force at the time of installation.
Guidance and information on plumbing systems is available from:
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your water company, which has a statutory duty to enforce the Regulations; and,
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the Water Regulations Advisory Scheme - www.wras.co.uk; telephone : 01495 248454