Hosting the side third in the Top14 table following a gritty away victory in South France earlier this season, Harlequins looked to solidify their spot at the top of Pool B.

The evening could hardly have started worse for the home side, with Castres catching Quins napping early, scoring a penalty within the first 60 seconds before following up with two tries within the first 10 minutes to claim 13 unanswered points. Fortunately for Quins, Castres fly-half Ben Botica missed both conversion attempts.

Quins found their foothold through the day's captain Alex Dombrandt, with the England back-row on the finishing end of some smart attacking interplay down the left touchline. Fly-half Tommy Allan hit the uprights with the conversion attempt, leaving Quins with an eight-point difference to make up.

Caught on the wrong side of referee Mike Adamson's whistle regularly during the opening quarter of the match Quins conceded a second penalty kick, with Botica slotting another simple kick to rebuild the away side's advantage, 5-16.

A rapid retort from Quins. Winning a penalty at the breakdown, the men in the Famous Quarters kicked to touch for a five-metre lineout. Allan sent a banana kick in behind the opposition defence, with wing Aguillon spilling possession – before being poleaxed by giant Quins centre Andre Esterhuizen – to allow outside centre Huw Jones to poach the ball and claim the Londoners' second try. No mistake with the conversion for Allan this time, reducing the difference to four.

Quins struck another blow five minutes later, this time through Tongan flanker Viliami Taulani. A lineout deep in Castres territory was again the source of points, with a powerful drive brought to ground illegally by the French side before Taulani followed up with a commanding short carry to bundle over. Allan dissected the uprights once more as Quins took the lead for the first time of the day, 19-16.

A far less positive final five minutes for the home side, with prop Joe Marler shown a yellow card for standing on Castre scrum-half Rory Kockott's hand at the ruck. Castres soon followed with their third try against the 14-man defence, with Kockott sniping down the left touchline. Botica's conversion restored Castres' lead, with a four-point advantage.

Quins were able to chip away at the score before heading into the changing rooms, with Allan adding a penalty as Castres were caught off-side.

Half-time – Harlequins 22-23 Castres

Despite playing with one less man Quins were the fastest out of the blocks at the start of the second-half, with Dombrandt claiming another try down the left touchline, running in an easy score after the Harlequin attack manufactured space after a kick in behind the Castres defence from wing Louis Lynagh caused havoc. Allan added his conversion once again, affording his side a six-point lead as Marler returned to the field.

Castres stole back ahead with two tries in reply to Dombrandt's brace, with wing Filipo Nakosi and centre Adrea Cocagi claiming their fourth and fifth scores to secure both a four-point lead and the try bonus point. Just over 15 minutes left to play, Quins 29 - Castres 33.

Once again the Mighty Quins treated their fans to a nail-biting finish, with a hat-trick score for Dombrandt treated to a lengthy TMO review with the clock in the red and the result at stake. However, Adamson adjudged the close-range effort to be valid to see Harlequins secure an unbeaten pool stage to the Champions Cup.

Final score: 36-33