UK Remuneration and police informants

[Forty knox]

A few weeks ago, it was announced (in the UK) that some local police forces actually paid 'informants' on an ad hoc basis. Intrigued by this revelation, I asked our local PCC (police and crime commissioner) if, when these payments are made, the recipient is liable for income tax (PAYE) and how the money was declared to HMRC. I was told (eventually) that, being a police informant, they are not required to pay tax on these 'earnings'.
Surely this isn't right and should be classed as 'tax evasion'!?