[spoilers] Lucas. Just Lucas.

[HellsBells]

That made a lot of sense, Bravo, especially the bit about 'what he was in love with was the idea of her'. There never was a great deal of love in the relationship, even Lucas admitted that they never really talked (in 9.8, and Sarah replied they had better things to do) and was mostly a physical relationship. I don't see the fact that Sarah was going to bug the flat as that much of a problem to their relationship, that sort of thing probably happens a lot, after all Harry bug his flat when got back from Russia. They even thought that they might be monitor at the very start of their relationship, hence they went to Claridges' Hotel.
As Byatil says though, it is interesting how Lucas is quick to trust those who hurt him, because even after he found out about Sarah's involvement with Nightingale he continued to care for her.