Originally Posted by swingaleg:
“But now they've almost disappeared yet they the Doctor still makes the Tardis take the form of a Police Box, thus drawing immediate attention to it when he lands in present day London..............why doesn't he make it look like a silver coloured car or a Port-o-Kabin ?”
The Chameleon Circuit (or "Cloaking circuit" in the TV movie) is used to adapt the TARDIS to its local surroundings (at least in the Type 40 model the Doctor has). When the Doctor landed in the junk merchant I. M. Forman's Totter's yard in c. 1963, the circuit got jammed and stuck in the shape of a public call box.
Now a couple of times since then the Doctor's tried to fix it. Most noticably was in "Logopolis" where the Doctor went to Logopolis to try and use its famous block transfer computations to try and heal the circuit. Instead he died and nearly ended the universe, so didn't quite go according to plan. The circuit is then irrevocably damaged except...
If you're getting nerdy about it, the 7th Doctor then
does fix the TARDIS circuit in the Virgin spin-off novels. This is because he's using a TARDIS from an alternate dimension which was never irrevocably destroyed (from the alternate 3rd Doctor incarnation). For a number of novels, the TARDIS changes shape appropriately until eventually the Doctor deliberately sabotages it back to its original form at the conclusion of an arc, demonstrating how things are back to normal.
Other TARDIS encountered, such as the Meddling Monk and the Master, have had their TARDIS cloaked into such things as an altar, a grandfather clock, and a computer bank as appropriate.
If you go by the novels, newer TARDIS went one better and disguised themselves as living, walking people that blended into their surroundings...