Organisation of Horsebox and Trailer owners (www.ohto.co.uk) seem good. I haven't had to use them for anything major but they were fantastic when our windscreen wiper came off in pouring rain on the M25!
OHTO is who I'm with and they were great when I needed to call them out because of a front blow out on the motorway. Yes they threatened to tow me home but I refused (was only 30mins from destination of W&W for 3 day show and home 2 hours in wrong direction
) so they managed to find a nice independant bloke (on a sunday) who could come out and change wheel. My issue was down to where my spare was located on the right hand side of vehicle....