If it’s over the boundary or on your land then it’s your problem, or after the meter box. Get them to check the exit side of the meter box thou which is still their responsibility.
If unsure or not happy mention to them that you’ll contact OFWAT and they should soon back down.
Water companies...
We do external water leaks via Anglian Water and as long as you sort it within a 30 day deadline they give you a leak allowance, more or less cover the cost of the loss of water. I would contact your buildings insurance as sometimes this is covered depending on what policy you have. Most will...