Harment from a vet?

Hi there, I owe my local vet �90 and have offered �20 a fortnight. He is not ok with this.He is also saying it will take 5 months to pay off but it wont at all �40 a month at 5 months is �200, go figure! my dog got sick on a bank hoilday monday and although I am on pdsa and I told him this on the day he treated my dog he insists that I didnt. I think the offer I have made is reasonable. He keep being very threatening towards me saying he will come to my etc etc. He proper shouted at me when I went down to discuss a repayment and this has now made me nervous of him and I dont feel I can go down there. He keeps calling my home phone. He said to me why did i take the dog to him if I knew I had no money and like I said I told him this before hand and he said it was fine but he doesnt seem to remember. anyway the whole treatment cost �10 for 2 injections and he charged �80 for call out. he also said he has a business to run which is fair enough but it seems he actually doesnt give a toss about the animals. as far as I am concerned he should be in a different profession. As I have made him an offer to pay him every fortnight surely he cant keep calling and threatening me? He just shouts and I cant talk back to him as I am so nervous. He also added interest onto the bill I dont know how right he is in doing this but my other half seems to think its against the law? How can I get him to back off me. I am female and feel very threatened by him. please help?