All newspapers - including the Guardian - have employed the services of private detectives to help crack stories - and when the PD's come back and say 'hey we've found this out for you.' the newspapers accept and use the information. The PD, of course, got the information using underhand means such as phone hacking....and therefore people like The Guardian and the BBC and C4 Dispatches can say 'but we had no idea they were using such tactics' (but, actually, they suspected as much but never asked)
The BBC were forced to admit recently that they too had employed the services of Glen Mulcaire - Glen Mulcaire hacked phones, pulled credit card bills, pulled phone bills, pulled medical records - that's what he did - and everyone in journalism knew it.
If you employed his services - as the BBC did - as other news organisations did - it was for those purposes