You said you've done an "automatic search"-type retune, so you have refreshed the tuning but had no improvement (which has cured this sort of thing in the past). I am therefore trying to diagnose where the problem is.
The first thing I wrote about above was to have a look at the transmitter web site to see what channel numbers you should be receiving the multiplexes on (typically in the range 23 to 69 if I remember correctly), these correspond with the old analogue channels. You should have 5 or 6 listed for your address, and you might be presented with alternative transmitters with different selections of channels.
Each of these channels carries about 10 DTV services (or 4 HiDef services), radio services, and data services. One of them will include ITV1. If you are getting poor reception on ITV1 you should be getting poor reception on the other services carried in the same multiplex. Note down the channel numbers relevant to your transmitter (I keep mine in the back of my diary) and note that they might be scheduled to change over the next year or so. If there is any doubt about which transmitter you should be receiving from, have a look which way your aerial is pointing.
Now press the "OK" button to bring up the service list, move the cursor to highlight ITV1 (which it will be already if you are currently viewing ITV1), then press the "i" button to get information for that service. It will tell you which multiplex (channel) it is being received on, and you can check whether that matches the list you noted. If not you should carry out a full manual retune as per the instructions.
If it does match, and you are still having trouble, the next possibility is that you have a connection problem. Use Menu >> Settings >> System >> Signal Detection, select the correct multiplex channel, and see what signal strength and quality are reported. I have never seen quality less than 100% if the signal it better than 40%, I got some funnies when it dropped to 10%. Any loss of signal should affect all the services on that multiplex.
Supposing you do all this and still not find a problem, I propose that there is something going on with your Humax and actually all services are affected, but the majority of your viewing/recording is ITV1 so you haven't noticed it on other services.
Instructions for manual tuning and link to transmitter information
HERE (click).