As you have posted to the European bulletin board rather than the UK one can you confirm you mean Liverpool, England?
All regional English police except The Met in London, are now using O2 Airwave. This is a digital trunked system and at this time there is no way as a hobbyist you can monitor it.
http://www.airwaveservice.co.uk/airwave351.asp
The Met are migrating to Airwave in sections.
Regional Ambulance Trusts are in the process of changing (slowly).
Fire & Rescue services will be the next major user to be added.
RSPCA recently announced their descision to use the service.
There are some military police using the system also
