It's only a criminal offence (Dangerous Dogs Act 1991) if a) the dog is in a public place
and b) is dangerously out of control
If the police think now that it was your dog but the incident wasn't in a public place (a place to which the public have access such as the highway or, perhaps and sometimes, a pub car park) then it's not a police matter From what you say, it wasn't in a public place.It may not have been dangerous either, of course, though it's not the law that a dog has one 'free bite'.It may be obviously dangerous and out of control but only have bitten once or not, in fact, have succeeded in injuring anyone !
There's a civil proceeding, not criminal, in the magistrates court too (Dogs Act 1871).It proceeds by way of 'complaint' by the aggrieved party. They have to show that the dog was dangerous and not properly controlled. The dog can be on private or public property.That's not a police matter (though they may be interested in the presence of a dangerous dog, that's purely a matter of interest, nothing more)
Thre's another crime which is committed by the kind of thug who goes into somebody else's private property with a dangerous dog but that doesn't seem material here!
I take it your dog is not 'a pit bull type' ! There's special provision for those, under the Dangerous Dogs Act 1991.