Richard Armitage has mentioned in interviews that things get very tense between Lucas and Sarah in Episode 6 and 7 so they are both still about

Thank heavens.....thought I'd read 'em all! Must have missed that bit.......

We know they're in the final episode because they're in the final episode summary.

Life of Wylie! Interview with RA “Sarah Caulfield is a really interesting character. I think I’d describe her as being a very complicated girl and I think it’s in episode seven, she gets Lucas in a bit of a mess.
“It starts off as a power struggle between two professionals. That then turns into a sort of sexual power struggle and then it turns into quite an honest love affair which screws Lucas up and actually screws Sarah Caulfield up a bit. The courses that they’re both on start to become compromised because of the fact that they do fall in love with each other."

I see that the cast listed in the summaries means little in terms of plot.....thank goodness.....

and thanks for the reassurances!