EXTREMELY FRUSTRATED

I booked a flight for my husband with Mango to fly out on Tuesday morning at 6am and return same day at 6pm. On Monday afternoon, after online check in, he receive a call to say that the 6am flight is cancelled and he can only fly out at 11am (take into account the meeting for which my husband travelled is at 1pm in PTA). So therefore we said that we want a refund because we had to re-book. They insisted that they will only give vouchers. Now we want to use the vouchers for an urgent flight, but according to the website all flights are sold out for the month of June. (take into account the voucher is only valid for 3months) I phoned the call centre and according to their agent, the system only refreshes on a Monday and I can try then only to book the flight on Monday and then I can see if I can get anything. The more I wanted to get a better answer, the operator was extremely rude with an attitude of "So what are you going to do about it?".