First of all, I don't know what you mean by the "baby bio" you are putting on the soil. What is it and how long have you been using it?
Secondly, this looks identical to what was on the base of the trunk when it was "previously" sick only now it appears to be coming up thru the soil (from the roots, perhaps?)
@ leatherback: When James initially "cleaned" this white stuff off the base of the trunk, it seems that the trunk surface underneath must have been very soft or even mushy in order for him to do that much damage with a toothbrush, don't you think? To the best of my knowledge, hard water scale (ie: lime build-up) would not cause surface mushyness and it's not that white, its usually a dull, ivory colour. Also when he "cleaned" the trunk base previously, the leaves were yellowing and dropping off and now he says the same syptom is recurring. I suspect there may a serious fungal infection in the root system. What are your thoughts on this?