Virgin Media TV is in the process of switching its HD channels to the more efficient H.264(MPEG4) video standard.
According to this thread on Cable Forum a number of customers have noticed that this can be a mixed blessing. Recordings take less space, which means you will be able to get almost twice as many recordings on your TiVo or V6 box*, but FF is now a more delicate beast and you have to be quicker with the remote as it flies through a recording much faster than before.
It appears that HD channel is being carried twice - once in MPEG2 and once in MPEG4 - which VM has confirmed is standard practice when switching to a new standard until all tests are completed.
*Obviously this only applies to recordings made since the switch to the new standard