To answer my own earlier question (#3071), in case the information is helpful to someone else.

I managed to get through to the agent who originally booked my ticket. D class grabbed for the code share, change made and within an hour confirmation that taxes had been recalculated £115 in my favour. No change or service fee.

Glad I could reach this agent, as a colleague of his who I spoke to earlier told me there was no D class availability. Sometimes it pays to HUACA.