Thanks for the advice everyone. In the end I called in, asked the agent to confirm I could get an FTV for a part-flown itinerary, FTV issued immediately but the confirmation email doesn't state the value of the FTV just a voucher code. I guess this will have to do...
Booking Reference: XXXXXX
Dear Mr XXXXXX,
Your future travel voucher is here. Thank you for your patience while we processed your application.
Voucher code(s)
125-XXXXXXXXXX
As you have started your journey, you can use this voucher towards completing your existing booking, or use it to pay or part-pay for a new booking. Note that we have cancelled all future flights from your existing booking so you will have to rebook these if you wish to travel on them. Your voucher has the unused value from your previous ticket(s).
Flights or holidays booked using your voucher must be completed by 30 April 2022.
How to use your voucher
Once you are ready to redeem your voucher, simply call us to book a new flight or holiday, at which time we will also confirm the remaining value on your voucher.