Source for chlorine/plain bleach

Tesco Thin Bleach, the cheap stuff in the white/blue stripe container.

Tesco Thin Bleach c) doesn't contain perfume.

Tesco Thin Bleach

Alternatively any farm supplier should stock Sodium Hypochlorite 10% (Red Label Hypochlorite) at about £10/25L

Does it have to be Sodium hypo? Just talk to your local swimming pool chemicals supplier who will sell you pure calcium hypochlorite in powder form.

Have you looked at swimming pool chlorine? That would seem to satisfy your requirements, though I'm not sure if that's hypochlorite.

Theo

As others have suggested, swimming pool supplies, farm/agricultural supplies or janitorial supplies.

I keep a drum of liquid chlorine (14/15% sodium hypochlorite) for the swimming pool, together with stabilised chlorine.

DON'T try stabilised swimming pool chlorine, it contains cyanuric acid as the stabiliser, which basically doesn't break down, and can only be dispersed by dilution.

Thanks to all for the replies. Sadly (!) Tesco and Sainsbury aren't immediately available shops for me (and I would rather not give them any business I don't have to, but that's another matter). The country store I tried doesn't do Red Label Hypochlorite, but I've found a decent looking online source of 15%.

I decided against using swimming pool tablets because I've read a suggestion that they the chlorine doesn't disperse as well or as quickly - the tablets tend to sit on the bottom. I decided against calcium hypochlorite mainly because of hassle factor - there's less need to "dilute" or mix sodium hypochlorite before pouring into the well, although I may do so anyway depending on what quantity I end up calculating I need.