Trading Standards departments come under county councils. So you should find them listed in the phone book under the name of the relevant council. However they often direct callers to Consumer Direct, which has regionally-based advisers. Call 08454 04 05 06..
Although it might be worth phoning the gardener, in the first instance, to see if you can get things put right, any further communications should be by letter (using recorded delivery and retaining proof of posting). As you might need to start a legal claim you should ensure that all of your correspondence clearly shows both your name and address and that of the gardener. All letters must be clearly dated.
If you can't get things rectified you'll need to claim a specific amount of money from the gardener, as compensation. You should either get several written quotes for the work or (after getting such quotes) get the work done and retain a copy of the bill. Then you should send a letter (as above) headed 'DEMAND FOR PAYMENT'. The letter should make clear the amount that is being claimed, and the reason that you are claiming it. Further, it should specify a specific time period for payment, with the threat of legal action if payment is not received within that period. (e.g. "TAKE NOTICE that, if payment of the said sum is not received within 14 days of this notice, I will commence LEGAL PROCEEDINGS to recover the said sum plus LEGAL COSTS".
If you still don't get your money, commence an online claim, here:
https://www.moneyclaim.gov.uk/web/mcol/welcome
Chris