Ok - it would appear that blowing the audio sockets with compressed air was, indeed, a hopeful coincidence afterall, as during the course of today the Ir remote control problems reoccurred with a vengance.
However, I am now pleased to report that my original thoughts of an external source of interference have proved to be the correct cause of the problem. By trial and error, I have located the source as being a vertically stacked, multiple 13Amp plug socket unit, which has an LED illumination located at its top.
Simply shielding this by way of placing household silver foil in front of the stack and, as an extra precaution, placing an additional layer of foil at the rear of the TV and the DVD units (being careful not to obscure any ventilation slats), it would now appear that the problem has been rectified with the IR remote controls working continuously without problem.
I hope that my experience may go some way to help others with a similar problem.