Turns out the answer is yes - I just did this for a second time yesterday.

It depends on the agent you speak to as to how easy this is. First time I did this involved quoting their own T&Cs asking for a manager etc etc before they finally, reluctantly did it.

Second time around I went in armed with having calculated the Avios and tax beforehand ready for the same battle I had the first time, but the agent only took 2 mins to get to the same numbers that I had and PNR was received after just 10mins on the phone.