Just another (and hopefully final) update.
The "new" graphics card arrived safe and well on Wednesday afternoon. So, with this in mind I embarked on a mission to obtain a can of compressed air to give the inside of the PC a good clean.
I went to PC World, and when I couldnt see what I was looking for, I asked one of the guys where you get your PC "fixed". He said that it was out of stock and that they didnt even have a can themselves. I was horrified when he said that they had resorted to "sticking the vacuum cleaner in" to get rid of any dust!!
Anyhow, on a happier note, I was able to install the new card. Turned on the machine and hey presto, it worked!! The tech from Dell even rang to see if everything was alright with the computer!!
You know, on reflection, Dell really isnt a bad company. The only problem is the stress caused by a PC failure and the amount of time spent on the telephone - waiting on a tech becoming available so that you can discuss it with them!! Resolution in my three cases has been acceptable! Or am I delusional!??!