Hi guys,
a very intersting thread. Well, I can tell you my experience too, but attencion! So it is in Germany. We have other temperatures like you.
Well, I think the best barbel-month in Germany is the August. In our country we have a cold witer from January to March and so the water is very cold. Then, in April the temperatures begin to get warmer but (for example this year) we can get snow in April too. May is a good month to start with catfish fishing, because they are very hungry from the past winter months, so you can catch nice fish. The June is good and in July the temperatures are around 20° Celsius. But in August, the temperature is higher (around 22° Celsius) and the catfish (barbel) eats a lot. In this month, you can catch our friend often at the bright day too.
Best wishes,
Uwe