Your aerial system needs improving.
In an ideal world, the elements of a TV aerial should be exactly the right length to resonate with the frequency of the incoming signal. Since each channel is on a different frequency, that would require you to have a different aerial for every channel. Obviously that's not possible, so all aerials have to offer a 'compromise solution' which means that they'll be better at receiving some channels than others. Your aerial is adequate for most channels but only 'borderline' for ITV3. (i.e. it provides a good enough signal for resolution of the picture but the signal strength is inadequate for reception of subtitles on ITV3).
Since you're generally getting good reception, you might well get a sufficient improvement in signal strength by simply using a cheap signal amplifier. If not, you'll need to upgrade your aerial to something like this:
http://www.maplin.co.uk/Module.aspx?ModuleNo=2 21124&&source=14&doy=search
(You should also replace the cabling at the same time).
Chris