Long loads

Hi guys , can anyone tell me what the rules are concerning extendable trailers , basically how many meters can you extend before you need escorting and police notifications given and what markings do you need to show eg beacons etc

Over 18.3 metres needs a notice and a second man or ■■■■■■ vehicle, (some say its 18.65 but we get a higher rate after 18.3 so i stick by that length) the trailer can be opened all the way to 18.3 without any problems, if over 18.3 its not law to have beacons on the back just a standard marker board will do