We've been running dehumidifiers in our house since moving in 7 years ago. Apparently they cost 8 times as much to run per day as a ventilation system. Although there is the lower outlay and not all houses are suitable for ventilation (sounds like yours is if you've had sales people around).
Just yesterday I had a Smartvent system installed. They were cheaper than the other suppliers because they don't employ sales guys to do the quotes. Installers do them so they know *exactly* what can and can't be done with your system. The installation took less than 6 hours and was very efficient. It's too early to notice much effect - although we had a southerly rip through yesterday so the bedroom got quite cold at night with the cold roof space air coming in! But I'm looking forward to not needing the dehumidifiers!