Thanks! So I didn´t have symbols, but added a few NOPs to locate the clear(). Nothing fancy, just a bit of address manipulation and a call to memset as you´d expect. I somehow expected some memory barriers, but I trust it more than my code 
So next I tried adding some global assembly, just a loop zero´ing the bytes, probably not my best assembly, but it works! Working out why is a bit beyond my rusty old brain. I also tried @memset(ptr, 0);
Might just be another phone memory access quirk, I already had this one: Strange multi-dimensional array issue
Thanks anyway, onwards again