I had perfect channel 5 reception - until I got a digital TV box!
Analogue Channel 5 is usually broadcast on one of two frequencies in the UK, either "channel 35" or "channel 37".
Some set-top boxes and video players come with the ability to "tune in" your television to their signal, and they often use, by default "channel 35" or "channel 37". (Normally you watch programmes from the set-top box using the SCART cable - the thick lead with the massive rectangular connectors on each end)
The feature is called an "RF-modulator", and usually somewhere on the menus of a set-top box there will be an option to disable this facility, or change the channel number to one that's not being used by channel five.
If you can't do this, it is possible to eliminate the problem with inexpensive aerial-cable connections.