In Broadchurch Season 2 Episode 6, towards the end, we see Lee and Claire check out a house. I imagine this is the house next to the Gillespies? So is this a flashback? The surrounding scenes were current events I think, so I am not sure. (Note: not a native english speaker, so some points of the series are hard to get for me)
No, this is not the house next to the Gillespies -- the back yard is much larger, that isn't the Gillespies next door, and Lee still has the cut on his head from being attacked by Ricky in S2E5. This appears to be a house that Lee and Claire are considering renting or buying somewhere else, possibly in Broadchurch, before deciding not to get back together.
It's easy to be confused, though: Claire just had a flashback to brown-haired Pippa jumping on a trampoline about two minutes previously, which is what the brown-haired girl is doing in the scene you mention. The implication is that Lee and Claire are haunted by their memories relating to the girls' disappearance.
(For future reference, this scene takes place in Broadchurch S2E6 at around 37:51 on Netflix US)