Airport express factory reset fail

Hi, my airport express (7.6.2) is showing on airport utility (version 6) as device not found. I have tried to do a factory reset - held the reset button whilst putting in the power cord, I get the amber light, then the blinking amber light but when I release it goes straight back to green. Have tried holding for shorter and longer periods - no difference.

Have tried connecting the express to my macbook air via usb cable and tried airport utility 5.6 - still not able to recognise it.

The device is showing a green light as if all is fine.

Any suggestions appreciated.

Thanks

Rob