Hi. I'm an overseas student just been awarded the PhD. I want to thank my supervisor for his support and I'm thinking of inviting him and his family for a meal or sth. In the past I occasionally had lunch with him, but I have not met his family before. Do you think it is a good idea to invite his family for a meal? What's the norm here in the UK?

I would say not. Normally people would just buy their supervisor a present like a bottle of wine or something. If you had met the family before because he brought them along to university functions or something, that would be a bit different, but still unusual.