Im glad im not knocking on your door then panther!!!!
Emamum if anyone tries to do the same again this year then what you need to do is ask them for their name, make a note of which party they are from and report them to the returning officer.
There are certain rules that people canvassing have to follow and if they dont follow them then they can get the party involved in trouble.
Basically he should have left form with you for you to fill it in at your leisure and for you to return it to the returning officer yourself. I have offered before to fill it in for someone and they signed it and i took it in for them but only with their say so.