Northampton down Toulon for second straight Champions Cup win
The hosts moved five points clear at the top of Pool 3 after a second straight win, with Toulon left floundering at the foot of the table following their home defeat by Exeter on the opening weekend.
Enzo Herve's early penalty gave Toulon the lead but tries from Tom Seabrook and England fullback George Furbank put Saints in control at half-time.
Melvyn Jaminet touched down shortly after the break for Toulon, but Furbank's second try gave Northampton a 15-8 lead.
The deficit was wiped out in a seven-minute spell as Herve slotted over another penalty, former Northampton second-row David Ribbans crossed his old side's line to score and Jaminet kicked a drop goal.
But the game changed when Toulon pair Matteo Le Corvec and Jeremy Sinzelle were both yellow-carded in the 76th minute.
Northampton seized their chance against the 13 men, with Lockett scoring the winning try after 12 attacking phases, which also secured a bonus point, before Fin Smith kicked the conversion.
Toulon's fellow French club Bayonne also suffered a disappointing night with a 12-11 home defeat to Glasgow.
Josh McKay's try in the 57th minute proved to be the final score of the game as Glasgow moved third in the pool standings, level on points with Exeter.
The Chiefs host Munster on Sunday in the last Pool 3 match of the weekend.