Siobhan was married to Dr Tim Hathaway. They had lived in Islington and then moved to Ambridge when he bought the village GP practice (now defunct). There was an intriguing reference in a private conversation between Siobhan and Tim to 'a new start', which was left tantalisingly unexplained. Siobhan was a freelance translator, working from home. One day in the autumn of 2001, Brian went round to Honeysuckle Cottage (which I think is where they lived - Adam and Ian are there now) to ask about translation services for Borchester Land. Siobhan asked him to have a glass of wine with her and it was somehow immediately obvious that they were about to have an affair. And lo! so it came to pass.
It was a very torrid affair indeed and obviously involved a great deal of deception all round. Eventually Siobhan became pregnant, which was probably deliberate but hadn't been discussed with Brian. At first she attempted to deceive Tim that he was the father, but he cottoned on. At that point they separated and Brian came under great pressure to leave Jennifer, which in the end he couldn't bring himself to do in the end. He didn't tell Jennifer what was happening, but was beside himself with pride at having produced a son. (Tim eventually went off with the vicar, Janet, and they live in Southampton now. He punched Brian at one point, and frankly who could blame him?)
Siobhan kept quiet about the identity of the baby's father. Those who were close to her began to realise it wasn't Tim's but nobody had worked out what was going between Siobhan and Brian. Siobhan's two closest friends in Ambridge were Elizabeth and Debbie. On the Pargetter twins' third birthday Siobhan was sitting with Ruairi, then about a month old, at Lower Loxley when Debbie arrived (Debbie and Elizabeth being very close too at that time).
Debbie quite suddenly recognised a strong resemblance in Ruairi to Alice when she was a a baby - or was there something with a hanky too? Anyway, the penny dropped, and she was absolutely devasted. Debbie had just found out that her own husband, Simon Gerrard, had been unfaithful to her and she had thrown him out. To find out that her beloved stepfather had done the same to her mother was too much. She confronted him and he had to admit it. She forced him to tell Jennifer, I think. Jennifer was devastated but decided to fight back and of course in the end she won. Brian stayed with her and told Siobhan it was over, at which point she left for Germany with Ruairi.
Debbie didn't speak to Brian for a long time. Some years later when Alice eventually found out (which I think was when Siobhan was ill and Brian was persuading Jennifer to have Ruairi to live with them) we had a re-run of that story, as Alice was so disgusted that she would have nothing to do with Brian for a long time.
All resolved now, of course, although I don't think Elizabeth and Debbie have ever got back to the way they were, because Elizabeth let Siobhan stay with them even though she knew the truth about the affair before Debbie did and Debbie never forgave her for what she saw as a great betrayal.
Peggy's role was to tell Brian that she wanted to horsewhip him. Wonderful moment. The whole story was absolutely gripping and I do think they bottled out in the end by not following through with a divorce, which would have been the more logical outcome. Ah well, we can't have everything.