I found that there's enough slop at the rear of the drive tray opening that the drive was sliding in and just slipping below the drive connector - which I only discovered by removing the main cover panel from the underside of the laptop. Once I could see what was happening, I was able to tilt the drive up using the moulded edge cover and slot it straight in. Before that, sliding the drive in there was a convincing sense of resistance and then applying a bit more pressure it seemed to slot into place. The moulded outer cover even seemed to align well. Rebooted and drive is now recognised. Another "check the power supply first" moment!