How far is it reasonable to travel for drinking water? A site with 3/4 portacabin offices approximately 5/10 minutes walking distance apart, only has drinking water in one office. Installing facilities in the other 2 cabins would be expensive and large bottles water fountains present their own problems. Do I have to provide water in every location?

If it is not practical to have drinking water immediately adjacent to the work site then you could provide drinking vessels, bottles, cups, or whatever at the site. HSE guidance states that drinking water should be 'readily available'. A 10 minute walk on a hot day is not really readily available in my opinion. The bonus is better productivity by providing immediately available drinking water. HSE guidance below.

A supply of wholesome drinking water should be readily available. Where possible, it should be supplied direct from the mains. If water is stored, protect it from possible contamination and make sure it is changed often enough to prevent it from becoming stale or contaminated. Where necessary, clearly mark the drinking water supply to prevent it being confused with hazardous liquids or water which is not fit to drink. Provide cups or other drinking vessels at the outlet, unless the water is supplied in an upward jet, which can be drunk easily (eg a drinking fountain).

A job-lot of bottled water from the cash and carry, in one accessible place, with more in store, come help yourself surely = readily available?

There will always be exceptions. When working on the railway we have had transient workers some distance from the site welfare facilities. In which case we have provided drinking vesels for them - job done.