Dispute with landlord regarding water damage

saajan_12

The crux is really whether you could / should have known it was leaking earlier than when you reported it. Whether that's by seeing the cracks, or seeing water running down from an odd place, etc. 
  --> If yes, then most of the bill would be down to you. The walls and floors of a building are expected to last a long time (20 years isn't long) so there isn't much betterment / depreciation to factor in. 
  --> If not, then its a LL repair entirely. 

dwty67 said:

My question is how do I work out the cost that we should be paying considering age of asset and what other factors should I be considering?

So there isn't really going to be a methodology to determine a % of the cost, just a matter for negotiation until / unless you can get a determination from the the deposit scheme arbiter / judge. You just need to think about how much £ you're happy to pay to avoid the bother of going to the deposit scheme and risk of it being higher. 

dwty67 said:

At the same time, I want to close this quickly so I have proposed to my landlord that I will propose an amicable solution and if that's not agreeable then we will approach TDS. The landlord doesn't want to approach TDS and wants to settle this by negotiation.

This isn't really the LL's choice - if you can't come to an agreement, then the backstop is TDS and court. If he wants to avoid those venues then he can offer a better incentive.