Whilst I'm confident of the USB-RS232, I did have a closer look at the DB9 - RJ11 cable and I'm not convinced the wiring is right. I've ordered some different bits from RS and will have a play with those before whining further!
In my experience, 90% of automation faults are cable not plugged in, cable broken or something needs a power cycle...
Sadly Joe, whilst I appreciate your support, you are mistaken. It seems a USB connection is offered on every Samba and Vision PLC except the SM35! Trust me - had it had a USB connection, I wouldn't have gone down the RS232 route in the first place...
Gonna have a play with cables as above, thanks for the ideas anyway, I'll come back if it's still no bueno...