Installing an outside light

I'm in the process of installing an outside light a process which normally I would consider simple. I'm using 1mm twin & earth cable have run the brown (live) from a 5amp circuit breaker in the consumer unit attached the blue wire to the neutral bar in the consumer unit and have covered the bare earth wire with green and yellow sleeving and attached it to the earth bar.
I've then at the other end of the piece of wire attached the blue(Neutral) and earth wires into two seperate contact blocks and the brown (live) wire into the back of a switch (the switch incidentally has only two connectors in it (live and switched live) from there I have connected another piece of twin and earth from the connector block (Blue and earth) and from the switch (Brown) to the light fitting connecting brown to brown, blue to blue and earth to earth the the connector block I've then wrapped in electrical tape and fastened up the front of the switch. The problem I've got is that when I've switched on the electric everything is ok until I flick the switch at which point the main circuit trips (not the 5amp breaker) what have I done wrong? I initially thought it was a problem with the wire but upon trying another piece still have the same problem? could it be a bad earth do you think? Help please, It's doing my head in good style

Try adams suggestion first,

Is your consumer unit a spolit load one, if so its got two neutral bars (look closely) and you have most likely connected to the wrong one

I assume it is an RCD which is tripping?

Try:
Turn off the supply,
Disconnect the outside light fitting,
make wires safe into terminal block.
Switch back on.
Does the ckt still trip when you flick the switch?
If the RCD doesn't trip, it is likely, you have a faulty light fitting.

What can I say guys, you were right it was a split load supply and I had wired it to the wrong neutral (doh!) Thanks so much for your help it is really appreciated