Thanks chaps for the responses. I did use the Extreme Outervision (or whatever it's called) PSU calculator last week before I ordered and was well within my current PSU wattage.
So, the outcome is I won't worry about my current PSU. I do not have any overclocking or major components drawing power on this machine - it's very much a processor, memory, graphics card, mouse, keyboard and DFP wheel only kind of affair.
I have some peace of mind from the responses though so thanks again.
JT