Shaun, did you look into getting a Chiller? It looks as though the Teco 500 is slightly underpowered. I've done some price checking in Germany and I can get the 500 for 489 quid, or the 1000 for 565 quid, which sounds pretty reasonable.
I think I have to go down to chiller route as this last weekend the temperature was 35 degrees in Central Germany, and the tank temp has already risen to 28.5, which is above the 28 limit I have set on the Controller, so the alarm is going off all the time. Normally it sits pretty well at around 27 degrees. I have to reset the limit to stop the alarm, however, this is the very first warm weekend and already I can see the temp rising significantly.