Saracens are not used to the rarefied heights of the Heineken Cup – this is only their fourth season in the 13 years of Europe’s premier club competition - so they may be excused for living dangerously. But they will not retain the initial leadership of pool four they achieved in Watford yesterday unless they tighten their defence.

They have, after all, to visit ambitious Biarritz on Saturday and, having opened a 16-point gap over Glasgow Warriors early in the second half, they almost contrived to throw it all away. The Scottish side have a persistent quality to their play, notably their abrasive back row, which served them well and helped them to a rush of 14 points in the last nine minutes that earned them a losing bonus point.

Alan Gaffney made no attempt to gloss over his side’s shortcomings. “We went out and did what we had trained to do and came away with five points,” the Saracens head coach said. “Our attacking game was very good and deserved that reward but our defence let us down. It’s not the structure, it’s individuals – a week ago we defended well against Bristol, there was no reason for that to happen.”

There are, though, some good tacticians in Glasgow’s ranks. Dan Parks, Scotland’s fly half, reads the game very well and Daryl Gibson brings an old head to the organisation of their back division. If they can contrive a way in which to give their flying wings a bit more space and if Bernardo Stortoni can curb his enthusiasm a fraction, they will justify the belief of Sean Lineen, their coach, that they need fear none of their pool rivals.

They must also tidy up their set-piece: the Saracens scrum gave Glasgow a thorough examination and a couple of lineouts went adrift but Lineen could not conceal his disappointment that they helped Saracens to, in his estimation, three of their four tries. That is a bit hard on the deft handling that, every now and again, enlivened the home side, most notably from the tight forwards in the build-up to Ben Skirving’s pushover try.

Glasgow might have scored in the opening minutes when Stortoni was brought down a metre short after collecting a cross-kick from Parks, and Lome Fa’atau went even closer. Parks did kick the first of his four penalty goals but Saracens responded through Rodd Penney, who cut through after collecting a bouncing ball and found Matt Cairns in support and Hugh Vyvyan available for the try.

Saracens, with Vyvyan and Kris Chesney at lock, have a mobile pack and their drive from a lineout preceded a run-around from Glen Jackson, whose pass allowed Penney to stride powerfully through a less than committed defence. But they could not close down Stortoni, who attacked Jackson’s missed touch with relish, found his centres in support and John Barclay finished well at the corner.

With half-time looming, Skirving scored at the back of a five-metre scrum after a delightful exchange in open play between the No 8 and Chesney. Parks hammered over a 54-metre penalty to open the second half and was just wide with another from the same range before a neatly contrived set-piece score for Saracens: the influential Neil de Kock went one way, Skirving passed the other and Kevin Sorrell doubled round to send Kameli Ratuvou over for the bonus-point try.

Yet Saracens could not force the issue, Parks kicked another goal and Hefin O’Hare found an opening where none appeared to exist. The replacement wing broke clear to the line and suddenly Glasgow were only six points adrift; Jackson gratefully kicked a penalty to give Saracens much-needed leeway but with the last move of the match, Scott Barrow and Barclay gave Alastair Kellock a richly deserved try.

Scorers: Saracens: Tries: Vyvyan (8min), Penney (20), Skirving (40+3), Ratuvou (52). Conversions: Jackson 2. Penalty goals: Jackson 3 (26, 37, 80+3). Glasgow Warriors: Tries: Barclay (30), O’Hare (76), Kellock (80+5). Conversions: Parks 2. Penalty goals: Parks 4 (6, 35, 42, 56).

Scoring sequence (Saracens first): 0-3, 7-3, 14-3, 17-3, 17-8, 17-11, 20-11, 25-11 (half-time), 25-14, 30-14, 30-17, 30-24, 33-24, 33-31.

Saracens: F Leonelli (rep: E Thrower, 63); R Penney, K Sorrell, A Farrell, K Ratuvou; G Jackson (rep: G Ross, 80+4), N de Kock (rep: M Rauluni, 80+4); K Yates, M Cairns (rep: F Ongaro, 80+4), C Johnston (rep: T Mercey, 80+4), K Chesney (rep: T Ryder, 70), H Vyvyan, P Gustard, R Hill (rep: D Seymour, 69), B Skirving.

Glasgow Warriors: B Stortoni (rep: S Barrow, 3-8); L Fa’atau, A Henderson (rep: Barrow, 54), D Gibson, T Evans (rep: H O’Hare, 19); D Parks, C O’Young (rep: S Pinder, 57); J Va’a (rep: E Kalman, 57), F Thomson (rep: E Milligan, 70), M Low, A Newman (rep: D Turner, 41), A Kellock, K Brown (rep: J Eddie, 70), J Barclay, J Beattie.

Referee: R Poite (France).

Attendance: 7,012.