Being there.........
South Coast
I was in a similar situation myself, firstly I tried to get someone to cover my watch, could you get on to your relief pilot and ask him to come down early? Maybe some sort of inducement, financial or otherwise?
Failing all that, in my case I called the prospective employer, explained the situation and luckily was given an alternative date.
I emphasised the loyalty issue and any reasonably flexible operation will respect that.
Good Luck
Leviathan