Owning your own home

FFS... long post incoming that I could do with some guidance on.

Tl;dr: there is a leak on shared supply pipe, but source is unknown.

Questions:

1. Insurance - a) should I notify both insurers? b) which one will cover it? c) does buying and paying for private H&C insurance prior to start date lock that insurer in?
   
2. Who's responsibility is the leak? a) if it is the shared supply pipe - is it the property it's located on? b) if it is on a property - that property's I assume?
   
3. Do water companies charge more when they fix it vs and indy contractor?
   
   

Facts:

• There is a leak on the supply pipe between 5 houses - call them No.s 1-5.
  
• Mid-June Water Co. sent letters out - followed by a 2nd letter to our tenants (1st wasn't rec'v), notifying them of water waste and need to identify and fix. This was apparently fwd to our letting agent, but Letting Agent has only just notified us as neighbour at No. 2 told them at the time he would email me - he did but it went into my my junk and was deleted.
  
• No. 2 has made efforts to contact Water Co. to find out if there was a leak. Finally in early Aug when No. 2 was on holiday, Water Co. fitted a water meter. Some time later No.1-5 turned off their water and the water meter showed water flow indicating a leak.
  
• No. 2 has spoken to an approved independent contractor, who gave a quote of c.£400 to find leak.
  
• Currently we have Land Lords insurance with buildings. I bought a new private/owner's insurance the other week, before I knew any of this which won't kick in for another couple of weeks. No. 2 spoke to his insurers when the issue first presented itself and they said until you know 1) there is a leak, and 2) you know who's responsibility it is you can't do anything.
  
  

My biggest concern is the leak being on our property, us being liable to fix, then being uninsured due to either; a) the insurers passing the buck between them, and/or b) not making them aware sooner.

The logical solution seems to be for No.s 1-5 to agree to get indy contractor in to source the leak then take it from there. Right?

It seems likely the leak is on No. 2 or us (No. 3). Based on; No. 1 being the victim of the leak (and having replaced all his pipework), No. 4 having an extension recently built (where builders claim to have inspected for leaks prior to work), No. 5 having recently sold and checked prior to sale.

Any help/advice much appreciated. Cheers!