No TVA invoice means you don’t have to pay any VAT but you risk alienating or even losing your gardener.
Quite frankly you’re nuts to be paying a gardener on the black because both of you are breaking the law . The chèque emploi system is specifically designed for help around the house or garden and will probably cost you less than you are paying now and will remunerate your employee properly.
For example, we have someone to help with home cleaning. Last month we paid her €135 for 12hrs work and paid her social security contributions of €61. Total outlay €196. But the government will refund us €98, a full half of our total outlay, by either tax credits or cash in 2019; so go figure. We employ someone quite legally for a net cost to us of just over €8 per hour while she receives just over €11 per hour, cash or cheque in hand at the end of each month.
Recently there have been a lot of new rules tightening up on cash in hand work etc. You say he is English so theres no language barrier. If hes in the EU & a brit there are issues coming for people due to Brexit. It is possibly that he recently registered for VAT & this is legit. Have you asked him? Did you have a nice working relationship? Is he local reliable decent etc, does he do good work? If the answers to the rest are yes & this is totally out of the blue, just ask him direct. He can answer direct- you can sort it out. You never know. If you start from a point of distrust it isnt going to improve. Its just a phone call. And if hes a total chancer he wont reply / face up to you. If hes legit it might just work out fine. Just a thought - and good luck