The 433 MHz systems typically have around a 3000 ft. range depending on the receiver's sensitivity rating, and the transmitter/receiver Seagull is using. Check out the link below. They have a good section on 433 MHz systems and the specs for range. See if Seagull lists the receiver's dBm sensitivity specs. anything around -100dBm is considered good. Linx systems chips are -116 dBm, which is very good. They claim over 3000ft range for the 270-450 MHz range. Check with your local regulatory agency for the use of a 900 MHz system in Europe. If you can use 900 MHz there, it would have far better range than the 433 MHz systems. At least 1 mile line of site; depending upon emi and interference from other 900 MHz systems of course.
http://www.linxtechnologies.com/

Regards,

Dan