I bought a new light and light switch for my living room and went to connect it.
There were 2 red (live) 2 black (neutral) 2 green (earth) in the ceiling and 1 red (live) 1 black (neutral) 1 green (earth) in the wall.

The light has a 4 way connection block (neutral, earth, live, loop) I connected the light first, 2 red to the live 2 black to the neutral, 2 green to the earth.
I connected the light switch, black to common and red to 1 way and the green still connected to the back box.
When I turn on the power the light is on, but the light swith is in the off position and when I flick the switch my fuse blows... can you help please ?

Where do you live?

Rotherham

Sorry that you have not gotten any responses to your problem.
Most of the people responding on this site are U.S. We of course do not know the electrical code or the wiring practices of the UK. We are therefore hesitant to give advise that could be dangerous.

I can tell you this much though. One of the cables coming into the light is of course bringing power to the light, the other is going to the switch. When you connect the red (hot) and the black (neutral) together you effectively extended the hot cable to the switch. When you turn the switch on you make a direct connection between the hot and the neutral. Therefore, you blow the fuse. By using the light connection block to connect all the wires, you also connected the light directly, in an un-switched manner.

You must take the power from the ceiling to the switch on one wire and bring it back to the light on the other.

This information should enable you to logically make the proper connections. If not you should seek professional assistance.

Harold is correct. We are both in the USA and electrical codes and apparently wire colors are different enough that our advice may be confusin or downright wrong, sorry.