Mum goes \link{http://www.walford-and-round.co.uk/stratford.htm\here}. The optician is called Mr Fantham (sp?). She says he is very thorough, takes about an hour over checking her eyes (mind you, she has a fairly complex prescription) and explains exactly why he is doing all the tests he does. Hope you like him if you go there.
Uncontrolled diabetes can cause blurred vision due to reversible changes to the lens of the eye. If uncontrolled for a long time of course you can end up with permanent damage and can eventually lose your sight. My dh has Type 2 diabetes and had blurred vision as a symptom before his diagnosis. If you're at that stage, I don't think an optician would necessarily be able to diagnose diabetes, as it would only be the permanent damage they would spot. So you'd need to get your GP to do a blood test. In fact, it's probably worth doing that before going to an optician because if you are diabetic and the eyesight problem is caused by high blood sugar, once you get your blood sugar under control your vision should change again.
HTH.