thanks for everyone's help - a special thanks to theo who helped me via PM. I have sorted this out now.
The steps I needed to take were:
1. download and install QHY5 drivers from
qhyccd.com (Windows 32/64 bit QHY5Drv.exe) - use the plain QHY5 drivers - not the QHY5V or QHY5T
2. Download and install
ASCOM platform
3. Download and install QHY5 ASCOM driver from
qhyccd.com
connect camera, start PHD, click camera icon bottom left, select ASCOM Camera (at the bottom of the list), then QHY5 in the ASCOM properties box that comes up.
At first it appears that the camera is still not working as the LED indicator is only very faintly lit - and with the other driver, this was a bad sign. However, start capturing in PHD, and the LED will light up brightly and blink in time with your chosen exposure duration. Happy days.
Hope this helps somebody.
Adam