Ian wrote:
I have installed a new bathroom light. (changed from a 60w bulb to a 38w
fluorescent) I have 2 cables coming into the light each containing a red,
black and earth core.

Part P ...

My old light had the 2 reds connected to each other using chocolate block,
the 2 earths to an earth terminal and each black to its own terminal.

Almost certainly this is the last light on a radial circuit (or a spur
off a radial circuit) and the two cables are (a) supply (b) switch.

If I do the same with the new light it remains on permanently.

Then you aren't doing the same ...

Your ceiling rose had 3 terms + earth, your new fitting probably only
has 2 terminals + earth.

I have swapped the blacks around and there is no change. (The
instructions with the light states to connect the reds to live, black
to neutral, and earths to earth terminal. This just trips the
breaker.)

It would do, because you are shorting the supply through the switch.

Can you help me - what have I done wrong?

You *probably* need to chocolate-block the two reds together and
unconnected to anything else. The black from the switch (switched live)
and the black from the supply (neutral) go to the L and N on the light.
The earths all go to the earth terminal. The black from the switch
*should* have a piece of red sleeving on it to show it's a live.

If in doubt, get a competent electrician.

Owain