Ok this is what I have found out. It comes under the Animal Welfare Act which came into force in March 2007, where it is an offence to let a dog be dangerously out of control in a public place. This leaflet gives you details.
http://www.defra.gov.uk/animalh/welfare/domest ic/ddogslawyouleaflet.pdf
It should be reported to the police (which I know you have done, but be more forceful and tell them it comes under the Animals Act 1971.
Also:
"Under the Dogs Act 1871, any person may make a complaint to a magistrates court that a dog is dangerous, or report the matter to the police. If the court is satisfied that a dog is dangerous and not kept under proper control, it may make an order for it to be controlled or destroyed.
The Animals Act 1971 provides that the keeper of an animal is liable for any damage it causes, if he knows it was likely to cause such damage or injury unrestrained.
Dog Control Orders
Dog Control Orders replaced Dog Byelaws in April 2006. Existing byelaws remain in effect until such time as a dog control order for the same issue is made on the same land. Further information on these Orders is available here. "
If you still have a problem with the police taking it seriously contact your local dog warden through the council and get them to advise you.