Finally some progress!

After many emails to GGL team, who always respond promptly and are very helpful, unfortunately the customer service team failed to respond to approx 7 requests over a couple of months for my request for 75% refund to be agreed.

GGL then took over and agreed that I would get 75% of the original cash and avios paid plus I've also had the GUF2 reinstated (so 200% refund on that part!). Avios and GUF2 applied to my account and just waiting for the cash to return on my AMEX card.

Took a while but seems like finally resolved. :-)