Assuming that British analog phones work similarly to those in the U.S. (I know your signaling is different, but here I'm referring to the basic technique of transmitting by modulating the current drawn by the off-hook phone), then try this:

Get an oscilloscope. Connect it across the phone line (float the scope's power-line ground first). Set the sweep rate to somewhere around 1 mS/div and the vertical sensitivity around 100 mV/div. Vertical input should be AC coupled to block the talk battery voltage. Take the phone off the hook, dial something to get rid of the dial tone and talk on the phone while adjusting the scope settings until you get a good view of the voice waveforms.

Now hang up the phone and make some loud noises in the room. An easily identifiable signal would be best, like a loud tone. If the phone has some sort of hookswitch bypass, the signal should show up on the scope.

Of course, if the phone incorporates a remotely triggered hookswitch bypass ( an "infinity transmitter" ), you won't see anything unless the bypass has been triggered. The best way to check on this is to familiarize yourself with normal analog phone circuitry and then take the suspect phone apart and look for anything whose purpose isn't clear or which doesn't seem to serve a purpose.

Since infinity transmitters are usually triggered by calling the phone and transmitting a tone to it just before the ringing begins, you could try to trigger a suspected one by calling the suspect phone and transmitting a slow sweep from around 300 Hz to 3 kHz and then repeating the scope experiment.

Or, if you're really paranoid, you could just disconnect the phone from the line when you're not actually using it. Of course, there are ways to use phone lines to communicate with and power other types of bugs, if you can gain access to plant one...