Sometimes people experience problems with the ITV/C4 multiplex (and also the one that carries five) because of the different transmission mode that allows more channels to be carried, but reduces the effective signal strength.
This can sometimes cause problems when the weather changes (due to the inversion effect).
If you rescan for channels on your box and don't get the ITV/C4 services restored, then you may need to get a new Class I aerial to get reception all the time. However, you should check your cables and connections are in good order first.
Anthony Chiocchi: Try replacing the cable from the wall socket to your TV first as it may be that the plug has become damaged (quite common, especially if the plug is situated where it easily be knocked). If that doesn't resolve it, ask your landlord or management company to get someone out to check and replace the socket. If the aerial has always been in the loft there is no reason why it shouldn't continue to give good reception provided all the cabling etc is in good condition.
StevensOnln1: Thanks for coming back to me. I will try your advice.
When we moved into the flat there was no aerial, so I put one in the loft, this was about 3 years ago and 90% of the time all signal is fine, other times when I play around with the cable I can get it to work but I suppose this could also be damaging the cable/socket even though I get the signal to work for a while.