Hello, I am a solicitor with 20 years experience. I will try to answer this.
What is the cause of the leak?
Most of the time it is bathroom issues, I've been told sink, toilet, bath... another time I was told the dishwasher was not properly installed.... the list goes on.
As a basic statement of legal principle it is the case that if your neighbour is breaching a duty of care to you and you are suffering reasonably foreseeable harm then he is responsible for compensating you for that harm. If he fitted the dishwasher himself or used it in an unreasonable way then he may well be responsible for that, if a professional fitted it then your neighbour will be responsible from point at which he had notice of the problem. As far as the bath etc. are concerned he will generally be responsible from the moment he has notice and does not fix the problem. However if he has taken reasonable steps to fix the problems and it is not his fault (eg some obscure issue to do with the building itself) then he will not be responsible. Do you have insurance for this?
Yes, building insurance, but tired of the process of keep claiming and the insurance doesn't cover loss of income. What is a recommended next step?
You should consider threatening legal action for recovery of the monies you have lost. You can do this through the small claims court here https://www.gov.uk/make-court-claim-for-money. You initial letter should set out all the problems you have had from the upstairs flat, that you hold them responsible and that you require them to pay a figure (justified in the letter) to compensate you for the loss. Should also liaise with your insurance company as if you bring a claim yourself without their agreement they may use that as an excuse to wriggle out of responsibility.
Okay, thank you.