All manufacturers max tow weight are generally based on the ability to restart on a 1 in 8 hill and take no account of stability. The general guidance for towing a caravan is that the laden weight of the caravan should not exceed 85% of the kerb weight of the car. Additionally, if you tow anything more than the car kerbweight, then you need to take an additional driving test if you passed your test after 1 Jan 1997.
The kerb weight of the Civic Sport diesel is 1407Kg so the max caravan laden woight would be 1200Kg. It would be unwise to tow anything over 1407Kg and possibly illegal if you don't have the correct licence. The 1500Kg tow limit is thus just a theoretical limit.
See this
http://www.direct.gov.uk/Motoring/Dr...073&chk=BIZx8Z
Brian