Well we do live in a "difficult" location. We can't get satellite as we live close to a wooded hill, and the satellite is below the tree line! In the other direction is Beacon Hill, and although its not very far away, it is obscured by another hill.
We had a new aerial installed shortly after we moved in, along with a distribution amplifier that incorporates a 4G filter. Initially we used to get drop-outs whenever it had been raining and a car went up the road! So we swung the aerial round to look at Stockland Hill. Then we got drop-outs whenever the steam train passed, depending on the state of the tide!
At this point I got fed up and re-aligned the aerial myself! I worked, briefly, as an engineer at a TV transmitter many years ago. Admittedly it was a 405-line B&W transmitter, but the laws of propagation haven't changed much in the intervening years!
My re-alignment got us a perfect signal on every channel, except the BBC SD channels. This didn't bother me as we had them in HD. The problem has only occurred since the frequency change. We now get everything with a very good signal strength and S/N ratio, except the HD channels dropping out for no apparent reason.
The fact that the TVI filter seems to have solved the problem leads me to the conclusion that there is an intermittent RF source somewhere along the path to the transmitter that is upsetting the Humax on a specific channel (HD). It could be CB, a taxi, police, ambulance, any number of things.
I do have a spectrum analyzer, but not the time to spend sitting staring at it for hours looking for the source of the problem. The filter seems to have done the job, so I'm happy for the moment.
I did consider buying a newer Humax, but the reports on the current line-up are not encouraging, with tales of clunky interfaces and other problems. Best stick to the devil I know! 
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Pete
