Get several quotes, from reputable gardening or landscaping firms, for the cost of remedying the problem. (Retain all of the quotes). Then send a written demand, for payment of the lowest sum quoted, to your neighbour.
Your demand must:
(i) be dated ;
(ii) show your name and address ;
(iii) show the name and address of your neighbour ;
(iv) make it clear that is a demand for payment ;
(v) specify the amount demanded ;
(vi) state the reason why the demand is being made (referring to the enforcement notice and preferably including a photocopy of the quote);
(vii) state that if full payment is not received within a specified period of time (I suggest 21 days) you will commence legal action ;
(viii) be signed by you ;
(ix) be sent by recorded delivery. (Not strictly necessary but a wise precaution, so that you can prove that it was received).
Retain a copy of the demand and await payment. If you don't get it, use the online claim system (provided by Her Majesty's Courts Service) to commence legal action:
https://www.moneyclaim.gov.uk/web/mcol/welcome
If you obtain a court order, but still don't get payment, enforce judgement by using one of the following options:
http://www.hmcourts-s...enforcement/index.htm
Chris