New version tested, transferred the driver chips over to the new board and all tests out well. I've also had a look at the IO expander and the I2C version can be used in place of the SPI one with no change to the board design, as I have added in address selector solder pads.

The SMD capacitors are much neater to the leaded version I had before. I have not run any testing of the current limiting, but the full range of step sizes are behaving themselves.

My plan is to squirt some heat sink compound through the holes under the drivers to provide better thermal coupling, without the hassle of soldering (try and avoid too much heat input).

I had an issue with the PolyZen unit short circuiting, may be a soldering issue, but this item is not critical so just jumpered it. Looking at the PolyZen its a double layer board, so you need to be very careful soldering (hand) that you do not create a short between the layers on the edge. Should be fine for those using re-flow ovens.

I've posted A3 files have a trace link that needs cutting if you are going to use the PolyZen unit.