directline has been the cheapest for me 3 years running. (23, 5years protected ncb)
but honestly, imo, the only way to get cheap insurance is tro try EVERY bloody company EVERY year. those comparison sites (for me) are a complete and utter crock of s**t. they come back about 200% more expensive than dedicated quotes. dunno why ANYONE uses them? i try them everyyear just incase they will be cheap oneday, but nope.
try and link policies (i.e. my dad is insured with directline, so we each get 10% of our quotes for having yours linked to another policy)