Your contractual duty is to report it (and in writing) to the person who engaged you to do the work.
Your moral duty is to report to HSE if you genuinely believe an immediate danger will be ignored by them.
Not true, this is nothing to do with contract law. Your contract with the landlord under electrical regs are a separate thing.
Although not explicitly stated in the HASAWA regs, you have a duty to report it to the HSE as a dangerous occurrence.
If you know, for example, that light fittings in a pub are live, you cannot just report to the landlord who you suspect will not get it fixed and go on your merry way. If you don't report it and someone gets hurt you WILL end up in the dock alongside them.
This is dynamic, it's not a fixed black or white thing.
If brianmoooore's pub hurt or killed someone and it could be proven that he'd seen it and figured he'd do nothing because the hassle of finding another hole to have a few scoops in was too much, he'd face criminal charges. Look at how the news article references almost the exact same sentiments expressed by the guy on trial who ignored what he saw.
In a private household, it's one of those 'can't do anything' cases - you can't report someone in their own house and if they get hurt it's their problem. But in a public place you are being negligent if you don't report it - not reporting it because you didn't want the hassle from the landlord proves that you knew he would likely ignore it anyway which compounds the negligence. A landlord willing to rectify the problem would have no problem with letting you put it straight there and then pending further works.
To just walk away and do nothing as 'not your problem' is a scumbag move if it's a public place.