gobbly, i would guess that ur cause is some kind of hidden allergy ... u should definitely have a full examination because diabetes may cause retinopathy and other nasty things..
there are some success stories of long term red eye treatment available at
Bloodshot eyes | Eye Care site for red eye symptoms and causes such as Chronic Dry eye Syndrome | Forum and Discussion Board for bloodshot eye and other eye disorders (ALTHOUGH EVERY CASE IS DIFFERENT !!!!), if it's not allergies then you may want to have a bunch of blood tests to reveal any systemic disorders
hope it helps, good luck
