When Lucy starts a quest to find her real father, the shocking results leading Dave to question the paternity of his and Rita's other children.
In anger at Rita's dishonesty in keeping Lucy's paternity and her relationship with the father a secret, and questioning Rita's fidelity, Dave sleeps with Sarah, who is angry and hurt when Dave starts post-coitally ranting about Rita.
Miss Moffett has also appeared in Peak Practice, The Bill, and Where The Heart Is.
Meanwhile Matt discovers that Sarah has cheated on him with Dave, and sleeps with Tanya as revenge, leading to a showdown at Dave's house.
Ultimately, Dave must slay the ghosts of the past, and decide whether to pursue new love with Sarah.
Last October Tennant parted ways with actress Sophia Myles after a two-year relationship, following her move to Hollywood.
They too met on set - when Miss Myles played Madame de Pompadour in a 2006 Dr Who episode.
The Tilers' lives are thrown into chaos when Rita is hit and killed by a police car; the children try to come to terms with the death of their mother, and Dave struggles to cope with raising them alone.
Dave and Sarah share a grief-stricken kiss, and are unsure of their attraction to each other.The pair both adopted a dressed-down look for a theatre trip to see Equus."They were certainly enjoying each other's company," said an onlooker."You can tell they get on well." Tennant recently split up with another BBC Wales staff member, Bethan Britton, 28, after a four-month relationship.Scroll down for more Miss Britton worked in the BBC contracts office in Cardiff, where Dr Who is filmed.At the same time he begins to doubt if the three other children are really his.