Well it's virgin media head end that's at fault then which I thought it was.
Are you saying your fault is now resolved?
The only other person I see reporting a problem, has (what sounds to me) different symptoms and their issues have now been moved into their own thread.
Which just leaves yourself with a fault - it may well be a VM issue, but unless they know about it - it's unlikely to get fixed. There certainly isn't a national fault with Channel 4 picture.
So what is the issue with your C4 picture? Is it pixellating and/or breaking up? Because that's usually a signal fault, which can very often be on customer's individual equipment and be specific to that user.
I think you're describing pixellation. This is often the signal of a signal fault, so if you've rebooted your box once, then there's likely a fault somewhere, so this will need VM to intervene. You're likely to need either an engineer visit or a replacement box, depending on what VM's remote diagnostics turn up.
First check the automated faults line for any existing issues in your area that could be the cause: 0800 5610061. If nothing's reported, assume VM don't know about your fault and ideally you'll need to call them.
150 from a VM phone, 0345 4541111 from any other phone - choose the options for "I have a fault with my TV service". Alternatively you can wait on here for staff to respond, which may take a day or so.
Do you have problems with ITV, ITV+1, or channel 4+1, as they are on the same mux. If you don't have you tried a factory reset of the box in case your problems coincided with the channel switch of 4HD from 141 to 104.
To factory reset go to Settings > System > Factory Reset.
It's only a partial system reset and not a complete factory reset.
You are given 2 options
Choose Keep Recordings, that means you won't lose your recordings and any planned recordings or series links you've set up, you will have to sign into your apps such as iPlayer, Prime, Netflix and ITV Hub again, also Match frame rate and Standby power consumption will go back to their default settings if you have changed them.