Thanks Pazza.

Update. I used the chat function today, and actually messaged with a person once the bot had determined my request was straight forward that I could be passed to an actual person. One can use the 'chat' online to request a new booking using the voucher, providing it's straight forward, and no other services are requested.

They have updated my booking to reflect the one I have just requested using my voucher, however, they have not added the full cost of my previous flight to the voucher total cost and I am missing almost £1k. As t's Easter, their refund dept cannot deal with it until after Monday.