As requested here is the Union/RMs definition taken from the Conduct code:
17 SAFEGUARDING CUSTOMERS' MAIL
UNEXCUSED DELAY TO MAIL
The responsibility for safeguarding the mail and giving it prompt and accurate treatment is one of the most important duties of all employees.
Various actions can cause mail to be delayed, e.g. carelessness, negligence, breach or disregard of a rule or guideline. Such instances are to be distinguished from wilful delay although they may also be treated as misconduct, and in more serious instances could also result in dismissal.
Furthermore, Royal Mail does recognise that genuine mistakes and misunderstandings do occur and it is not the intention of the Business that such cases should be dealt with under the Conduct Code beyond counselling for the isolated instance. The key is that employees should do their job to the best of their ability and, if in doubt, ask for their manager's advice and guidance.
WILFUL DELAY
Wilful delay of mail is classed as gross misconduct, which if proven could lead to dismissal. The test to determine whether actions may be considered as wilful delay is as follows:
Deliberate action taken by an employee that causes mail to be delayed is called wilful delay. Where proven, such breaches of conduct can lead to dismissal, even for a first offence; indeed Wilful Delay is a criminal offence and can result in prosecution.
And from Appendix 3 Which is a repeat of the above but posted for information.
5. SAFEGUARDING CUSTOMER MAIL (ROYAL MAIL CONDUCT CODE)
UNEXCUSED DELAY TO MAIL
The responsibility for safeguarding the mail and giving it prompt and accurate treatment is one of the most important duties of all employees.
Various actions can cause mail to be delayed, e.g. carelessness, negligence, breach or disregard of a rule or guideline. Such instances are to be distinguished from wilful delay although they may also be treated as misconduct, and in more serious instances could also result in dismissal.
Furthermore, Royal Mail does recognise that genuine mistakes and misunderstandings do occur and it is not the intention of the Business that such cases should be dealt with under the Conduct Code beyond counselling for the isolated instance. The key is that employees should do their job to the best of their ability and, if in doubt, ask for their manager's advice and guidance.
WILFUL DELAY
Wilful delay of mail is classed as gross misconduct, which if proven could lead to dismissal. The test to determine whether actions may be considered as wilful delay is as follows:
Deliberate action taken by an employee that causes mail to be delayed is called wilful delay. Where proven, such breaches of conduct can lead to dismissal, even for a first offence; indeed Wilful Delay is a criminal offence and can result in prosecution. 
your quite right disgruntledpostie, most posties in are office would be done for wilful delay with all the lightbulbs, your supposed to get 2 days 
to deliver them but most frames are piled high with them ,there is just no way you can do them with all the other crap we're delivering at the moment. 