Quote:
"(8A)If an authorised officer of a litter authority proposes to give a person a notice under this section, the officer may require the person to give him his name and address.
(8B)A person commits an offence if—
(a)he fails to give his name and address when required to do so under subsection (8A) above, or
(b)he gives a false or inaccurate name or address in response to a requirement under that subsection.
(8C)A person guilty of an offence under subsection (8B) above is liable on summary conviction to a fine not exceeding level 3 on the standard scale.”
[Section 88 of the Environmental Protection Act 1990, as amended by Section 19 of the Clean Neighbourhoods and Environment Act 2005]
So failing to provide your name and address to a litter warden is a criminal offence, with a maximum penalty of a £1000 fine. Litter wardens have no powers to detain anyone but they can alert the police (and, in relevant areas, CCTV operators) that two criminal offences (i.e. dropping litter and failing to provide personal details) have been committed by the offender, allowing them to be apprehended.
When local authorities have 'major clean-up' campaigns running, it's not unusual for them to involve the police so that they (or their PCSOs) can be available to support their wardens in their duties.