If someone else's
negligence results in damage to your property, you're entitled to receive compensation for any financial loss you suffer as a result of it. (A dog is, in this respect, regarded as 'property' and an injury to it is regarded as 'damage'). So, if your claim was to be heard by a court, you'd need to be able to establish that the spaniel's owner was negligent.
Given that he was aware of the animosity between the two animals, it would seem reasonable to have expected him to keep the spaniel locked up until he was certain that you'd left his premises. So, by failing to do so, it would appear that a court might well rule that he was negligent.
If you can't reach an amicable settlement, your only way forward would be to send a formal letter of demand to the guy. That letter should:
(a) show your name and address ;
(b) show the name and address of the guy you're sending it to ;
(c) be dated ;
(d) include the word 'demand' (as in 'demand for payment') ;
(e) clearly state the amount demanded ;
(f) show a breakdown of how you've arrived at that figure ;
(g) state the reason behind your demand (i.e. because you've suffered a loss through the other person's negligence) ;
(h) include a statement indicating that if payment isn't received within a specified period (I suggest 14 days) you'll commence legal proceedings ;
(i) state that if you need to resort to legal proceedings, you'll ask the court to add on your costs to the order made against the guy ; and
(j) be sent using Royal Mail's 'Signed For' service.
If you then fail to be reimbursed, you should commence legal proceedings using the HM Courts and Tribunals Service's online system:
https://www.moneyclaim.gov.uk/web/mcol/welcome