Ah - thanks; I have discovered that although I have assigned half my RAM (8 Gb of 16) in “host memory limit in MB for tiling” the "minimum amount of memory in MB for a single buffer in tiling takes precedence - at least, that is what the tool top says if it set to a positive non-zero number.
The trouble is, I cannot set this value to zero (or a negative value). Trying to overwrite it, or using the plus/minus buttons, I cannot get a lower value than 2. How to override this?
Maybe i just set it to a huge value - 1024MB?