Problem with neighbour

Tenancy Types and Management

I let a flat in a block, all flats have one allocated parking space. My tenant has just moved out, so i am redecorating. My tenant let a neighbour use his parking space as he didn't have a car, but the neighbour has 3 or 4 (some untaxed and 'junkers', plus visitors.

My problem is they keep on parking in my space. Everytime I go there I tell the neighbour and he apologises and gets the car moved, but it is getting on my wick!

I have put up a sign saying private parking on My spot.

I don't want to let it rule my life and become a nut about it, but he just doesn't stop using iot or his guests using it. He always is quick to move and I really don't want to make a bad atmosphere.

My next step will be to write to his landlord, through ther Management Company, but am looking for any 'low impact' alternatives.