Information provided by Mr George Barker, national coordinator for urban development in towns and cities, English Nature:-

Vermin, in the form of brown rats, house mice and large flocks of birds (e.g. pigeons) must be able to feed, breed, rest and nest in an area if they are to become established. With regard to wildlife areas, typically those urban parks on the edges of towns, these conditions do not exist for vermin to become established.

Vermin are more likely to appear in buildings where food and warmth, combined with breeding, resting and nesting places are abundant than in a town pocket park or nature reserve.

Example:-
The lawns of the White House in America, here there is a problem with vermin because:-
1) excessive feeding of birds and squirrels provides an abundant food source;
2) warmth from surrounding buildings help sustain populations during the winter;
3) the sewage system provides an environment for rats to breed.

An edge of town wildlife area will have none of these attractant attributes. If there are any  road drains they will be for surface run off, performing the same function as roadside ditches, entirely different from a main sewage system.

The subject of vermin becoming established in wildlife areas is not a problem, confirmation of this can be obtained by contacting local Environmental Health Offices and Pest Control agencies.

Reference to any wildlife handbook will show that conditions do not exist for vermin to become established in a wildlife area and English Nature have not found it necessary to produce supporting literature.