Cheers guys for the comments.
Bryan...There was an outer layer of bark which I have completely removed...it was very flaky on the surface, but became tighter and harder just a couple of milimetres in. Once it was all removed, underneath it there was a hard, dark brown skin, a bit like the hard, brown skin that you sometimes see inside Brazil nut shells. I managed to remove nearly all of it...just a few odd signs of it here and there.
Underneath the hard, dark brown skin is the inner bark which is still intact, with the exception of the little bit that's missing in the final photo. I took this small piece off because it was very loose, and sure enough, it revealed an even spikier texture beneath it. I think that this layer is called the cambium layer, and it's just about impossible to shift because it's like concrete. I tried digging a dental pick into it and all that came away was a tiny amount of fibre.
What makes the thought of any more work on it just too daunting, is that the more bark I remove, the rougher and spikier the burr becomes and after the 3 hours that I spent digging away the outer layer of bark, my hands are in one heck of a state.
Les