After effects of water leak

[Bottleblonde67]

I've been in my new build property since 20 December 2017.
Last October (2019) my boiler stopped working. It turns out the seal around the flue hadn't been installed properly so there was nearly 2 years of water coming in through the roof and I didn't know.  All sorted out. 
On 24 December 2019, I noticed water staining on my hallway ceiling, took photos reported it to builders, no response.  I didn't follow up in January or February as I was unwell, then we went into lockdown.

Gradually since then the damage has spread, I contacted housebuilders again - they told me not our problem you were out of warranty by 4 days.  I got my own plumbers in, they only went so far before they said you need to get the plans of where the pipework is in your property. 
The housebuilders refused, so I called them out on Twitter within 5 minutes  of the post I had contact and a promise that I would get a visit from the Site Manager.  They came, blamed the plumbers. 

The plumbers attended today and sourced the leak, it wasn't their fault, the carpenter installing the skirting board had used a 50-60 mm nail to go through less than 30 mm of skirting and plasterboard straight into the pipe so in my opinion I would class this as a latent defect. 

Has anyone got any advice as to how to proceed with this in terms of getting the housebuilders to sort the damage and ensure that I haven't been left with any long term issues that may affect me selling my property in the future.  Any advice would be gratefully received.

[Midwest]

If the developers refuse to rectify, I would get some legal advice.

As you say this is a fault you couldn't possibly of foreseen during the warranty period, and with the issues caused by COVID, they should be making some adjustments. Perhaps speak to your insurance company, see if they can offer advice. Do you have any legal cover in your insurance policy?

[Bottleblonde67]

Thanks for the response unfortunately no legal cover with my insurance.

I have told the developers who have agreed to fix the cosmetic damage that I will be employing my own independent building surveyor to ensure that the damage isn't more structural as I do not trust those that are on their payroll to be fully impartial and they will be picking up the bill for the cost, not that they have agreed to it.

It isn't just the nail through the pipe, but the water I had coming in previously for 2 years due to the flue not being fitted correctly and having a hole in my roof.  I was under the impression I had bought a house not a sieve and that is pretty much what I told the CEO in an email on Friday.

[New Home Expert]

Why have you not named the housebuilder as a warning to everyone else?
"4 days out of warranty"  - Really? what an excuse!

You do need to get your roof timbers checked as water penetration for that long even with treated timbers may have long term issues.

[Maxell]

The plumbers should spray paint the floor where pipes are under when the pipes are not next to a radiator. Once the walls are plasterboarded it's not possible to see. The nail plugs the pipe initially but over time it corrodes and starts to weep out. I guess you need to prove it was done during installation. 

[Midwest]

If thats the case, and the builders won't play ball, and you've have had other damage to your property, I would speak to my insurance company. If you can provide evidence of the nail in the pipe, they might wish to take on the claim against the particular contractor.

[JM1973]

Best thing is to go to your insurance company, ask for help/advice and maybe even try to take a loan for legal help. This case is serious and it shouldn't be left like that.