At all the places I've taught, the conversion from T/L to student is about 1-2%. The vast majority of people that have trial lessons, get it as a Birthday/Christmas present, and no matter how you try to sell it, they're not going to want to learn how to fly. It's even less likely now, when they find out that the average cost of a PPL is £5-6,000.
For the majority of people, the T/L is on par with a Balloon flight, tank driving, a day at the motor-racing circuit and everything else in the 'Acorn' and 'Red Letter Days' type catalogues. The object is to make sure they have a good time, tell their friends and spread the word. I've had some people come back on a regular basis (every year) to have the same T/L. It's an opportunity for them to go flying, take a photo of the house and enjoy themselves. If they have no intention to learn to fly, there's no need to ram it down their throats. Give the customer what they want.
Obviously, if someone shows any inkling of enthusiasm, they get as much encourage as possible to take it further. Of the people that I've come accross learning to fly, most have come in to club with the intention of doing it. Not many have been the sort that, only after the T/L did they decide to fly.