Can't change my flights?

I am trying to change my return flights (for April) to a later date. When I open my booking on the site, it indicates that its a one way flight. On my booking confirmation, it's return flights.

I need to change these flights, and the call centre is not operating.

I also sent an email (as directed my Mango's website) to receive a voucher for my flights. It says the voucher may take 7 days to be issues. It's been 13.

Really terrible service with no solution.