The Irish province squeezed past Harlequins in their Round 3 meeting in Belfast last week, with scrum-half John Cooney kicking a late penalty to give Ulster a 25-24 success.

Now they must go to Harlequins for the reverse fixture, knowing a victory would further extend their advantage over ASM Clermont Auvergne – who play on Sunday afternoon against Bath Rugby – at the top of Pool 3.

“Basically, it can be two totally different games,” Reidy told Ulster’s website. “You might play them at home and put on a relatively good performance. Then when you go in front of their home crowd, it can be a totally different game.

“At the end of the day, I just don’t think we can be that complacent going over there and we can’t expect teams to come here and roll over.

“You saw how much energy and enthusiasm they brought to that first 20, 30, 40 minutes. We’ve got to do exactly the same over there.”

The 30-year-old forward believes Ulster will have to improve at the breakdown if they are to notch a crucial away victory in London.

“We left a lot of points out there and probably got dominated at the breakdown a little bit by Robshaw and the other back-rowers, so it’s important we go over there and really take it to them,” Reidy added.

“They slowed up our ball and got a lot of turnovers. We certainly created opportunities here and made some good carries, but then couldn’t finish those two-on-ones and we were a bit sloppy with a few offloads.

“We had a pretty harsh video review and we’ve got to improve on a lot of things. We have to front up over there.

“When that last pass goes wrong, it’s quite frustrating, but on the flip side, if two or three of those went to hand, it could have been a different story.”

Ulster Rugby flanker Sean Reidy has warned his team-mates not to be complacent when taking on Harlequins at Twickenham Stoop in Pool 3 of the Heineken Champions Cup on Friday night.