It is often assumed that the people who would be most likely to benefit from a mains water filter are those living in London and the south east. But when it comes to the very hardest water in Britain, some of the locations may come as a surprise.

According to Scaleguard, the town with the hardest water in England is in fact in the south west. Swindon measures 349 mg/l of calcium. If it was much harder, a replica of Stonehenge might form in the sink.

While Milton Keynes is second on 308, another West Country location, Bath, is just behind on 307. Slough and Hemel Hempstead are also over 300 mg/I, and all the towns and cities with the hardest water are in the south west or south east.

That, however, does not tell the full tale. The wider map of areas over 300 mg/I covers most of East Anglia, but also places further north, where water is supposedly soft. 

In fact, there are some very hard water areas up north: Around Chester, East Yorkshire, the Tees Valley and coastal areas of Durham, for example. Conversely, there are some soft water areas down south, especially Devon and Cornwall. 

However, most of the south and Midlands has water that is at least fairly hard, even if not very hard. Southern Water accounts for this from the fact that 70 per cent of its supplies come from chalk aquifers and just seven per cent from reservoirs, although the proportions vary markedly from place to place. 

By contrast, United Utilities, which supplies the north west, relies very heavily on reservoirs in the Pennines, Lake District and north Wales (most of Wales has very soft water). 

Since the Lake District is the wettest place in England and two of the lakes (Thirlmere and Haweswater) were turned into reservoirs - with Haweswater alone having a capacity of 84 billion litres - soft water is not in short supply. For others, however, it is time to get that filter.