How to cope with lying

My daughter is 8. Her sister and father have ADHD. I had no concerns about my daughter until recently. When she was younger, she was prone to extreme tantrums and would lash out but it was not that bad. However, I’ve noticed in the last year or so that she lies. A lot. And she will not back down, even when she has been caught out lying. She struggles a lot with friendships at school and often falls out with her friends. One mum sent me a message one night asking if we were emigrating in the half term cos that’s what my daughter had told her child. We have no plans to emigrate. She has very extreme reactions when things don’t go the way she wants them to, and she starts getting very confrontational and aggressive.
I don’t know how to get her to stop telling lies. She lies about so many things, things that seem irrelevant and there’s no need to lie about. She will say one thing then say something contradictory 5 minutes later and deny the first thing. She will not ever say sorry. It does not matter how many times I try to explain the importance of saying sorry if you have hurt someone, even if it is an accident. 
Does anyone have any advice on how to deal with this? I don’t know who to ask to get help with it. I’ve mentioned it to the SENCO at school, who is her teacher, but the SENCO isn’t interested because they don’t see the behaviour at school.