Most likely the conditions aren't related. However, if you're having any trouble with joint pain or with inflammation of the eyes (conjunctivitis), it would raise the possibility of Reiter's syndrome. Reiter's can follow NGU or chlamydia and causes various combinations of arthritis, conjunctivitis, and a fungus-like rash that often appears on the feet (especially the sole). But if you're not having those other symptoms, the rash probably isn't connected with your past NGU.
For further reassurance, you might recheck with the GUM clinic or your GP, or perhaps ask either of them to refer you to a dermatolotist. (Your use of "GUM" for genitourinary medicine clinic suggests you are in UK, yes? Most GUM specialists in UK are familiar with Reiter's syndrome and what the rash looks like.)
Good luck-- HHH, MD
The CDCs material on syphilis might be the sort of thing your friend was thinking of. Here it is . . .
http://www.cdc.gov/std/Syphilis/STDFact-Syphilis.htm