I am guessing that you live in the NW, receiving TV from the Winter Hill transmitter, & that the problem with the Humax PVR's tuners is due to the signal changes on 4 November when the first phase of DSO (Digital switch-over) took place. My impression is that the Humax machines have been more prone to losing channels than some other makes, & this thread may suggest some things for you to try -
Problems with Humax PVR-9150T The complete DSO takes place on 2 December, when the digital signal should be strengthened, so you may get the missing channels back then, if you don't succeed in the meantime.
One thing in your post puzzles me. You refer to having to disconnect the Humax in order to use the Freeview tuner in the TV. But if you connect from the Aerial Out socket in the PVR to the TV's Aerial In (& by Scart from the PVR to the TV) you shouldn't need to disconnect, or even turn off, the Humax in order to use the TV's own tuner.