I've lived down here for 7 years and seen the rents rise with the house prices

I'd say for what you want you'll have to pay about £700 pcm. Anything cheap gets snapped up before it even gets advertised.
I've since moved out of Newquay to get a better property for less money. Rather have the money in my pocket than a greedy property agents.
Start & Co I found to be a bunch of arses. About as helpful as a chocolate fireguard

Even when they knew I had more than enough income to pay for the rent they still treated me like shit.
David Ball are quite good though but not cheap.
I used
www.cornwallhomeseekers.co.uk when I moved down as they had no agency fee back then, probably do now. They were very very nice and friendly.
Hope that helps a little.
OP
P.S: I may be looking at a house share soon though to get the rent down, anyone interested?