Saracens scored two tries to one as they edged to a narrow win against Ulster in the Heineken Cup on Saturday.
Saracens (12) 18
Tries: Skirving, Yates
Pens: Jackson 2
Ulster: (7) 10
The hosts dominated possession in the first half but only led 12-7 after tries from Ben Skirving and Kevin Yates with Tommy Bowe crossing for Ulster.
After the break Glen Jackson kicked two penalties to deny Ulster a bonus point for losing by seven points or fewer.
Humphreys' late kick for Ulster had narrowed the gap to just 15-10 before Jackson's last-gasp effort.
Jackson and Humphreys both struggled with the boot, missing two penalties apiece.
After last week's 19-10 win for Ulster in Belfast, Saracens coach Steve Diamond had promised his side would "smash" the opposition back in London.
The hosts duly dominated first-half possession and took the lead when Skirving latched onto Ben Johnston's precise grubber kick to power over for a try converted by Jackson.
However, within a minute Ulster hit back as Bowe scooted over for a converted try after Neil McMillan had charged down Castaignede's attempted drop goal.
The hosts struggled to break down the impressive Ulster defence but they managed to add a second try when Yates drove over for an unconverted score in first-half injury time.
In the second half Saracens continued to dominate possession but it took until the 63rd minute before they managed to add to their score through a Jackson penalty.
With the final whistle in sight Humphreys landed his only penalty to make it 15-10 and Ulster nearly levelled the scores when Kevin Maggs went over in injury time.
However, the try was ruled out because Bowe had a foot in touch when he made the final pass and the last word went to Saracens as Jackson's second penalty ensured the visitors went home without even a bonus point.
In the other match in the group leaders Biarritz, who are three points clear of second-placed Saracens, won 38-24 against Treviso in Italy.
Ulster coach Mark McCall:
"My understanding is you can go upstairs for the try scorer but not the person who gave the pass. In that respect the officials got it wrong, but we don't feel cheated because it wasn't a try.
"It was very disappointing to concede that penalty under the sticks at the end. It will be difficult for us to qualify now. Ideally we needed to win and if not we needed a bonus point."
Saracens boss Steve Diamond:
"I was very happy to beat Ulster because wins are hard to come by in this group. If we get a bonus point against Treviso then we have a chance in Biarritz.
"Biarritz is a difficult environment to play in. We beat them at home but it's different going down there - I did that with Sale and they put 40 points on us."
Saracens: Scarbrough; Bailey, Castaignede, Johnston, Vaikona;
Jackson, Bracken; Yates, Byrne, Visagie; Raiwalui, Chesney;
Vyvyan, B Russell, Skirving.
Replacements: B Russell for Scarbrough (79), Bartholomeusz for Castaignede (65), Dickens for Bracken (56), Seymour for Raiwalui (76), Broster for B Russell (24), Randell for Skirving (58).
Not Used: Cairns.
Sin bin: Yates (22).
Ulster: Cunningham; Bowe, Maggs, Trimble, Steinmetz; Humphreys,
Campbell; B Young, R Best, S Best; Harrison, McCullough; N Best, McMillan, Wilson.
Replacements: Topping for Cunningham (66), Boss for Campbell (52), J Fitzpatrick for B Young (52), Ferris for N Best (33), Caldwell for Ferris (63).
Not Used: Brady, Larkin.
Referee: Joel Jutge (France)