i have got a dimmer switch downstairs that turns the upstairs light on and now want to fit a dimmer to the light upstairs, but the light is now flickering

[matte24]

[Clanad]

One of two things... if the upstairs light also has an on-off switch in the circuit, you may have installed the wrong dimmer switch. It will (at least here in the U.S.) take one marked "two-way" which will allow dimming only from that switch and on-off from the upper switch.
Alternately, you may have installed an inexpensive dimmer switch. With it on for a few minutes, place your hand over the switch plate and see if it's overly warm. You may also hear humming comming from the switch (don't know the name of the tune though). These are indicators the switch isn't a more substantial switch. Finally, are any of the bulbs in the light fixtures your trying to dim other than incandescent. You really can't dim most fluorescent bulbs and most halogens require special switch's.
Here in the U.S. our electricity supply is 110-115 volts whereas, as I gather, the U.K. standard is 240v, but the physics should be the same.
Best of luck!

[Hymie]

If you have a light (or lights) controlled by two switches – as is normally employed in a stairway, allowing the light(s) to be switched on or off by either switch – and have replaced one switch with a switched dimmer. This allows the dimmer switch to switch the light(s) on and adjust the brightness via the dimmer switch. The other switch can switch the light(s) off or on (at the set dimmer position), but does not allow the brightness to be controlled by that switch.

Without extensive changes to the wiring between the switches, you cannot simply add a second dimmer – so that you can control the on/off and lamp brightness from either switch – it does not work like that.

Replace the second dimmer with a switch, and hopefully things will work as before, otherwise you may have damaged one of both of the dimmer switches.