Hi Martin,
I sympathise with you, one of the reasons I switched to Kodak Readyloads. I know this won't solve your problem because you are using HP5, sadly Ilford don't produce a similar system as Kodak and Fuji.
When I was using holders I tried almost everything, short of working in a vacuum, and no matter what I did I still got the occasional 'pin hole', small black speck(s) in the print. If they were small enough I was able to knife them out on the print.
Good luck,
Trevor.