Seems simple enough.

I've connected my Toshiba laptop to my Samsung LCD TV using a monitor type lead with seperate audio connection but simply pressing F5 which looks like it should switch between the laptops monitor and external screen doesn't have any effect. (I know the lead is correct because it's worked in the past).

I did have this problem at work once when connecting to a projector but eventually got it to work although it had to be either laptop or projector not both.

Do I need to set up the display to allow me to switch between screens?

John.

shouldn't be as simple as pressing F5. on my laptop, i have to press fn F7, fn is the little button on the bottom left of the keyboard, there should be one on yours too

also, you need to press the input button on the telly, til you get round to one that says PC, or something

Mine is Function button and F4

As blake said, you'll need the function (fn) button if the f-key has a coloured icon on there (ususally purple/darkblue/grey).

Dependent on your graphics card (I'm assuming you have integrated on the your laptop) you may also have to fiddle in the control panel.
Control panel -> Appearance and themes -> Display properties -> settings

Good luck

you may have to go to control panel to enable dual monitor function on those hot keys.....
had to do that a few days back

Make sure your TV is on the right input channel. It should make it clear on the back of the TV what input you are using. Most laptops will auto sense a monitor and switch on, if not you need to find out what function key combo your laptop uses. Also make sure you don't have a bent pin on your cable. Good luck.

or with windows 7

windows key and p

Fn F4 on my HP
Choose AV on TV if it doess not switch auto