after my previous failure to solder down the micro controller correctly, I bought a couple of slightly cheaper ones (I didn’t need all the power in the original anyway) and tried again. After some careful work it seems to turn on!

• I can connect via an STLink v2 interface and openocd 
• I can toggle a pin on and off
• I can connect via USB in DFU mode

I need to write more software to properly put it through its paces, but so far so good - despite the soldering looking a bit questionable in places!

As to where I went wrong last time, I think the answer is too much solder and not enough patience!