Told?

There's a marked difference between telling someone something and enforcing a decision. One assumes the maturity to act upon a request the other presumes that without a big heavy stick, such a request will fall upon deaf ears.
BBC.

The NHS will have a legal duty to be honest about mistakes as part of an overhaul of the system in the wake of the Stafford Hospital scandal.
The move is part of a package of measures in England to put patients at the heart of the NHS, ministers said.
There will also be a new ratings system for hospitals and care homes, while changes to nurse training will be piloted.
It comes after the public inquiry claimed patients had been "betrayed".
The harrowing neglect and abuse at the hospital between 2005 to 2008 which led to needless deaths has already been well documented.

Fine, they have a legal duty, now who is going to make them do it? Who is going to be responsible for inculating a culture of honesty rather than the one where everyone covers everyone else's arse? I rather suspect it's going to be the managerial staff whose target culture led to the scandal in the first place.
Yes, you can make it a criminal offence to cover up after a colleague, but it only becomes a criminal offence if the mistake is noticed, or that someone decides to act upon it.
The problem with the NHS... (Scratch that I could be here all week and not even make a dent in that)
One of the problems with the NHS is the culture of the staff. Frequently it's an us vs them attitude in which the staff working under pressure lose sight of who is supposed to be at the pointy end. Mistakes get made and instead of honest appraisals, the tendency is to either ignore, or cover up the error. After all, get it wrong and people sue, or get sued. Yes the injury lawyers r us culture that the last government set away has not helped, that factor is always going to be in the back of managers minds if or when they learn of mistakes, errors and malpractice.
Hospitals and health naturally became a target of choice for a lot of people in the something for nothing culture that we now appear to have. This led to an increasing culture within the NHS for covering up mistakes simply to avoid being sued, such a culture already being there to an extent in the struck off list of doctors who had to be some sort of mass murderer to even be summoned to an enquiry, never mind lose a job.
So, I don't believe that anything will change even with a new code of practice. The culture itself has to change, the same applies to the likes of the police, fire brigade and ambulance services to name but a few. How this could be achieved i do not know, but telling them won't work, it never has.