Spoke to someone on the EC Silver line who dealt with Avios changes and they said I would be able to use another Barclays voucher for the return if and when it became available. Was surprised enough that I asked them to confirm this again and also to make notes on the booking to confirm the same and have noted down details of the call in case it comes to that. They were quite clear this would be possible, let's see what happens if and when it comes to actually booking!