Go straight away to your GP.
This is important in case you have a retinal tear, but it is more than likely, as John Lambert says, vitreous fluid.
However, most GPs will refer you to an emergency appointment with a hospital eye clinic to be on the safe side. All new floaters/flashes should be investigated. I got a hospital appointment the following day after seeing my GP.
I speak from recent experience and I am OK.
Unfortunately, Shadow Man, it is usually age related!!! It does not affect your site, but you do have to get used to it. The brain does learn to ignore it in time.
See here
http://www.rnib.org.uk/xpedio/groups/public/do cuments/PublicWebsite/public_rnib003660.hcsp
ps I am not over 65!!!!