Hi Hilly
Everything I have read from reputable scientific sources suggests that echinacea is completely ineffective, unfortunately. As are other traditional cold preventers, notably high doses of vitamin C and zinc.
The best way to avoid catching colds is to stay away from places where you might encounter cold viruses (crowded places, especially those where children congregate, are the worst), and wash your hands a lot since the viruses are transmitted just as effectively by hand contact as by having someone sneeze in your face. It might be worth having a flu jab too. This won't prevent colds, but will protect you against the three most serious strains of flu expected this winter. As a fit runner you're unlikely to get a flu jab free on the NHS, but Boots and other places offer them privately each autumn.
Even more importantly, make sure your immune system is in top form. This isn't achieved by taking things that come in bottles. It means commonsense healthy living - plenty of good food, exercise (but not overtraining), dealing with unfinished business promptly instead of letting it hang over you, and being prepared to rest if you do start coming down with a bug.
I don't take echinacea or any other supplements, I probably encounter more snotty noses that most people, and I had no viral illnesses between June 2001 and June 2002 - and those I do have tend to be trivial things. So Robokidney and I are an anecdotal study of two.
Hope that helps!
Cheers, V-rap.