What on EARTH???

If the pipe disappears under the floor when it comes into the house then definately check out the bath waste.. Or even kitchen.. check for any holes anywhere big enough for the pipe..

If you empty the barrels you may find some more clues.. You could always use a drill pump to empty them.. I might be inclined to have a look under other slabs as well..

 

One tip if I may - remove as much of the pipework back to the house as you possibly can, and you may even wish to glue/weld/seal that tap into the 'drain' position (assuming you can't remove it altogether); if you have any redundant pipework that you cannot remove (because it is through a wall or underfloor for example), then you need to make it rodent proof - to do this, I would suggest squirting expanding foam quite well up into the pipe, followed by copious amounts of wire wool (rats and mice HATE wire wool), and then finish off by either bunging it with a large rubber bung, or (preferred) build up a seal with weatherproof all weather sealant (available from Wickes etc for around £7 a tube).

 

You're welcome - we had a hell of a time with mice in our last house; they were somehow getting into a cupboard that was right in the middle of the house, and I couldn't for the life of me work out how. I eventually worked out that they were getting into the house up a pipe under the floor that ran from the electric consumer board which was housed in the cupboard all the way out to the back garden, which the previous householders had used to carry electrical cables for their pond pumps and water features.

Once I had stuffed it with wire wool and weatherproof silicone, I then spent a couple of months trapping and dispose of the ones that were left indoors until it eventually cleared. It was a real pain, as every surface in the house had to be bleached within an inch of its life a couple of times a day.

 

I'm glad you have confirmed that, I think it's quite certain it was a system to collect and recycle used bath water. It is a bit baffling how an old lady could have used it, maybe it was installed years ago when she (or her hubby) were fit and able?

On the face of it re-using water is a good idea and could reduce your water bill if you are on a meter. So just have a think whether you would ever use it in the future before ripping it out.

If you do decide it should go, I think it would be straight-forward. You could bucket out the water (make sure the left hand cock is switched off so it doesn't refill again!).

Then dig around a little and lever them out. Depending on how deep they are you are going to need a quantity of soil to backfill the holes.