I was desperate too, earlier this year, and I resorted to Milben Ex - expensive and lethal- you need to wear face mask. I bought it here:
www.jsrservices.co.ukAnother product widely recommended is a liquied called Poultry Shield, but I haven't managed to track it down locally, and by post it's expensive because heavy.
Anyway, I used the milben ex twice, 6 days apart, having first removed the roofing felt and internal fixtures like the perch support. Seems to have worked, but I intend to keep up the efforts every few weeks. The milben goes a long way I've found, so in the long term not as expensive as I first thought. good luck Declan