Thanks for the responses so far folks.
The current one-way is business promo, I fare class, so unlikely to have any option for cancellation. Re-faring seems a possibility though, as it looks like the total price for the outbound and inbound come in significantly cheaper.
I'm led to believe I won't be able to recoup that cost if the decide to be nice to me, but at least it might save me shelling out on the full cost of the extra leg.