I had a similar £1000+ quote from a company (or their supplier) when I bought the Ribtec, as the trailer bearings were slightly noisy and there were no braking components at all...
I looked around fairly sure I could do it cheaper and probably by myself! Because I had no examples in the axle and wasn't entirely sure on all the components, I just bought a complete new backplate for ~£70 each, stripped the components and fitted to the existing back plates. Example: Al-Ko Brake Backplate Assembly 230 x 61mm LEFT HAND
. Cables and bearings easy to find and not very expensive.
I think I spent around £350 on parts to completely refit a braking system to a twin axle Alko trailer, including the main braking cable to the handbrake.