Step 1: Obtain an EXACT figure for the cost of replacements, with documentary proof. (That might be in the form of formal written quotations or it could simply be by, for example, getting hold of the Argos catalogue).
Step 2: Write to your neighbour, demanding the full amount as compensation for his negligence. As you may need to show a copy of the letter to a court, it's extremely important that you ensure that your letter is dated and that it clearly shows both your name and address and that of your neighbour. (Also, even though you could simply hand your letter to your neighbour over the fence, you should send it to him by recorded delivery, and keep the proof of posting). The letter should clearly indicate that, if payment is not received within a defined period of time (I suggest 21 days), you will commence legal proceedings.
Step 4 (if necessary): Commence a legal claim against your neighbour, using the online system provided by Her Majesty's Courts Service:
https://www.moneyclaim.gov.uk/web/mcol/welcome
Chris