A couple of ideas Iceman,
Did the service tech change the scart lead as well as the STB? I may be wrong but I seem to recall someone mentioning that some of the scart leads being used now do not have the control pin connected which would mean that the TV will never see the auto switch signalling.
Following on from what Jem said as well as audio issues the socket for the VCR does not contain the signaling so if the leads are connected the wrong way round th TV will never switch.
Lastly go into to the settings menu on the STB (press guide then move the curser box until settings on the top right corner is selected
) select visual setting and ensure that the scart control is set to on, if it already is then set it to off come out of the menus switch the STB to standby switch it back on and then reset the scart control back to on.
As Rob says somtimes they can be a bit flakey but that will normally reset it.
hth