I've managed to run avrdude from a cmd prompt to interrogate the Promini with optiboot loaded.
Two things have shown up: firstly I had to set the baud rate to 19200 otherwise it produced the same sync errors I experienced with FTDI programmer. Secondly the Hi fuse byte is set to $DE and according to the Atmel datasheet setting bits 1&2 high allocates 256 bytes for the bootloader?? My understanding is optiboot occupies 512 bytes.
I'm out of my depth from here. Hoping someone may be able to tell me if this is likely to be causing corruption to the bootloader.
Also what do I need to do to change the hi fuse byte in optiboot to say $DA. Any advice greatly appreciated.