Apologies if this has been discussed elsewhere, but did some googling and couldn't find anything.

I flew LHR-JNB-CPT in Club in December and was downgraded on the South African leg as the plane was economy only. They gave me a printed voucher for 800 ZAR.

Does anyone have any experience of redeeming these things? Does it have to be on Comair flights or can I use it with BA in general? Also does it have to be on the phone or can I just input the 6 digit alpha numeric book reference into a voucher box?

Any help greatly appreciated!