Hello Stuart.bothwell , greetings from healthcaremagic.com
I have gone through your clinical case scenario and it is inadequate for me to come to an significant conclusion how ever i will try to throw some light on your query . If you are seeing prominent blood vessels / veins and left testicle pains without touching or moving it --then most probably it can be an early case of orchitis or rarely torsion of testis, you need to visit your physician and please get a sonogram done for both your testis and scrotum. I assume the pain is not associated with any trauma as you have not mentioned such in your history. Do you have any problem/ or feel increase in pain while passing urine or stool. Please do not neglect your symptoms and go see a doctor at the earliest. Take care