I have been informed today that when driving a boat trailer in Italy you will need a a fully reflectorised square panel 50cm x 50cm. These can be bought at service stations or on the attached link (thanks to Helen Selden for finding this link)
http://www.agentfiamma.co.uk/fiamma-carry-bike-alu-plastic-signal.html
So you know The AA says:
Any vehicle with an overhanging load (e.g. carrying bicycle at rear) must display a fully reflectorised square panel 50cm x 50cm which is red and white diagonally striped, a fine may be imposed if the sign is not displayed. This also applies to vehicles such as cars/caravans carrying bicycles at the rear.
The police can also do spot checks on cars in Italy so you need to carry a proof of ownership document (original) and a letter from the owner stating you can drive it if a company car plus a tax disc and insurance.
And in France you need high visibility jackets and 2 breathalizer testers
(note from Helen Selden – I think it’s a high-viz jacket for all people in the car and not just the driver)…
John Hanson
The Flying Fifteen Blog 
Not being silly, but where do we put it without obscuring the number plate! Is hanging down swinging about going to be acceptable? Or on the trailer itself which is not the reamost part of the trailer?