thanks corporate-wage-slave

Also, I thought booking an open-jaw/companion voucher without incurring a charge via the helpline would be straightforward, based on other posts here.

But the agent said I would need to book the outbound companion voucher RFS myself (on the website), get the PNR and the agent would add the inbound segment afterwards. If I wanted the agent to do both outbound and inbound, despite the fact that I can't do it online it would incur a charge.

On the plus side though, he was happy for me to book the outbound while I was on the phone and hand over the PNR for him to do the rest.