Scottish Law Questions? Ask a Scots Lawyer
I am a solicitor in Scotland. The landlord is talking nonsense. The root cause of the problem was a faulty shower fitting for which he is responsible. The fact that you didn't notice the leak is irrelevant. It was a latent defect and couldn't be easily seen behind furniture. The landlord is entirely responsible for everything other than damage caused by the tenant and you didn't cause any damage. The landlord has to keep the property to the repairing standard and the faulty shower fitting would breach that standard. You can't be held responsible for the loss incurred as a consequence of his failure to keep the property to the repairing standard.
Negligence as a legal test is characterised as doing something or omitting to do something that no reasonable person would do or not do. Would a reasonable person have noticed having regard to the furniture and layout? I would say not. Your landlord is at it!
I think you should hold out until this insurance company prepare a report. Even then, as I have already said, I don't think that you should be held responsible for what are essential latent defects which you could not reasonably have known about.
You should prepare your own inventory and provide it to the letting agents. Under no circumstances should you meet the landlord face to face after the way he has behaved. They can't charge you for items on an inventory that has never been taken to start with and the photos are irrelevant.