Thanks for the comprehensive replies.
Here is the link to the company offering the NAD upgrade:
http://www.fideleaudio.co.uk
At only £80, the upgrade seems very good value for money considering the work that is done. Also, I am bearing in mind that I have been advised that I would need to spend near £500 or more on a new amp that would yield worthwhile improvements in sound quality over the classic NAD 3020e.
Also, I have found another firm that can offer significant upgrades to my Marantz CD63 player. Depending on the options I choose, this will cost up to £220.
http://www.chevinaudio.co.uk/index.htm
So, put the two together and a potentially "new" system for £300 which may cost upto £750 to match if buying new kit?