The experienced jockey was banned for six months by France Galop after he tested positive for a prohibited substance at Saint-Cloud last October.
Having finished fourth in the Group Three Solario Stakes at Sandown on his latest appearance, the Khalid Abdullah-owned Purser was the 4-9 favourite for the Injured Jockeys Fund EBF Novice Stakes and readily scooted nine lengths clear of his rivals.
Havlin said: "I was looking at the day saying there was no pressure, but when I pulled up after the line I thought 'thank god for that'.
"Nothing went wrong. They're never winning until they've passed the line, but he's done it well.
"It was obviously a bit of a plan to get me to come back on a winner and it's nice of Frankie (Dettori) to step aside and let me ride him. He didn't take too much persuading to have another day off!
"I've been doing as much fitness work as I can do, but there's no comparison to the real thing."
Havlin voluntarily stood himself down in January and despite maintaining his innocence and making several appeals in a bid to clear his name, the suspension stood.
The ban, which officially took effect on April 4, was ultimately reciprocated by the British Horseracing Authority.
"What happened has been well documented. It's been a long road, but we're back now," Havlin told At The Races.
"Now I've got the first one out of the way, hopefully it's onwards and upwards. I'm staying here for the winter. I've got a lot of catching up to do, and I'm skint!"