Hello Meurig,
That is a very interesting point you make, I have to say that I have never had any real doubt about their reliability though.
If you have any concerns in that respect, I think that I'd actually write to them and see what they have to say for themselves if I were you.
I know that the Boers compliled their own rolls actually in the some of the camps and I have seen two examples of these myself in the past.
I would also suggest you contact the SAMHS, they may be able to help you and in addition I think I would speak to SADF documentation centre in Pretoria as well.
In the case of ABO recipients, both Vorm B and C are very interesting.
But there are still a number of roads to go down here, at Kew, in addition to the roll David has mentioned i.e WO108/303-305, take alook in WO108/368-369.
There is some interesting memoranda in CO537/403-409 regarding the actual camps and if you are interested in the foreign volunteers, such as the German Commando and the Irish Volunteers, FO2/824-826 does make for very interesting reading.
Good Luck with your research Meurig,
Regards Frank