I've never had the honour of doing Mnt Ventoux for real but I used to very regularly virtually ride up in on the turbo. If tacx are to be belived the resistance of the turbo would realisticly vary according to the terrain/gradient as you pretended to ride up the climb.
I can't remember the gradient ever going beyond about 10%, which for North Yorkshire is nothing. Its a completely different sort of climb to what you get in the Dales. Ventoux is all about being able to maintain a high power output not about how strong your legs are. (Hope that makes sense).
The lower the gearing you have on the bike then the slower you can go and still pedal at a reasonable cadence. The slower you go the lower the wattage you need to maintain.
Most of the climb will be approximately 5%. At this sort of gradient you should be able to happily stay seated and spin the pedals at a reasonable cadence, say 80rpm. If you can't then you need to change the gearing. The hill is far too long to stand up to climb or mash your way up in a big gear.