At the beginning of this series, it showed that they had all fallen out and then it went back six months to show the events leading up to it. We can see that Kate and Richard are falling for each other so that will be one reason for the others to fall out with Kate.
Can't remember all the details now about Trudi's husband. She thought he was dead, kept getting mysterious phone calls and then eventually he showed up.
The chap Trudy is with now isn't her not-dead husband, its her daughter's friends Dad. The reason they fell out is what we will find out in this series.