Have you done anything to the outside of your eye recently, such as scratch it?
Did you notice this blood vessel enlarging over the past few days / weeks?
Do you have retinopathy of any degree in that eye?
Have your BG levels and diet been stable or have you had ups and downs in recent months?
As it's not affecting the inside of your eye and your vision, it may not be seen as an eye emergency, but I would seriously consider contacting NHS Direct (or whatever it is they're now called) and perhaps getting an appointment to see an eye specialist.
It could be nothing, but it's almost always best to seek a specialist's opinion.