Fault on lighting circuit

[sparkz39]

Hi, I have a fault on a lighting circuit in a commercial premises. Power is fed to the switch and a single switch wire and neutral goes on to feed five downlight bulbs and then onto 3 LED'S. If I turn on the switch neither the downlights or LED'S come on and the MCB cuts out, although if I turn the MCB back on leaving the local switch closed the 3 LED'S work! Any advice would be appreciated. With thanks