I had been struggling to find guide stars when using the above combination and was confused as others indicated that there should be loads of stars visible. In a chat about the Sky-Watcher guidescope mount over on AstronomyShed a member advised that he normally increased the gain on the QHY5 all the way up to 95%. Bam! Tried it and now I have lots of lovely stars to guide with.
Thanks Keith (AKA Aardvark)