Posting this incase it helps others with similar symptoms (DVB-T digital in UK)
I moved from an (alternate brand) Digital TV-card to a Nova TD-500 about a year ago, and suffered regular signal dropouts and many missing channels.. unusable in dual tuner mode, but reasonable in diversity mode. However that was not the point of the dual tuner!
Believing the signal was too weak, I tried a couple of aerial amplifiers which made it worse - however after upgrading to the beta driver on this forum I found one of the muxes started working under dual tuner mode.. Using the signal strength tool, quality/BER was coming up between 0-20% on the other muxes... this made me suspicious that it could be analogue crosstalk from the Hannington transmitter (aerial should be picking up Crystal Palace although it is further away)
That led me onto trying to decrease the overall signal with a 6db attenuator, and the result was astounding - all channels are now coming up with 100% quality.
So for some people with issues on reception, it could be a nearby analogue transmitter causing the problem. Why this affects the Nova TD-500 and not other TV cards and STBs, I'm not sure - but this is maybe worth adding to a FAQ somewhere.