From a VOSA point of view then an occasional favour to a friend with no more than a share of diesel money changing hands is probably ok, esp if they are there (ie you are doing it cos they are your friend and they dont have a trailer but you do and you were going anyway). Mirroring the recent changed guidance from VOSA on those offering friends with their horse a lift to shows in their horse box/trailer, this is deemed ok in the circumstances above (it had been a problematic grey area for a long time.)Need to check on insurance implications tho.
But if friend isnt there, or done regularly, or done for so many people that they couldnt all be close friends, or taking more than a share of diesel money....then you would be into hire and reward and you really dont want to go there unless they are very good friends and you have deep pockets and a love for paperwork