I have an FVRT200 and noticed a blocky picture and clicking occurring more and more often. When I looked at the same page in the set up as above (Aerial Alignment) I observed that the errors level for both tuners were reading above the maximum level. I changed to another channel from the same broadcasting station (Crystal Palace) and saw a good signal with no errors. I telephoned Digifusion (08702405005) to ask them what to do and was informed that the tuners only have an expected life of two to three years and since my box was older than two years I should buy a new one.
(I guess one of Bat-man's two tuners had 'gone')
When looking up information on the web about this issue one is informed to check for electrical interference by unplugging all electrical equipment and then plugging it back in one at a time, especially those which contain thermostats (fridges, washing machines, heating systems) but also that the issue could be a thinned fuse (so anything) and that the faulty equipment could be in a neighboring property so you may need assistance from them (well that's going to be too difficult for most people in an urban area). I guess it is worth checking whatever you reasonably can in this way before purchasing a new box (I personally think that the tuners should be replaceable!)
Happy viewing.