[X-Newbies] Time Machine in LIMBO

[Clive Read]

I run Leopard 10.5.6 on a 13" white MacBook and was VERY pleased with  
Time Machine Back-Up, but a few days ago I noticed that the anti- 
clockwise clock was turning in the tool bar, but when I checked the  
message always said

Backing Up: 0 Kb of 190.0 Kb

I am backing up to a Conceptronic CSM3PL 500 Gb external hard drive.

If I do Apple+I after I have clicke on the external drive's icon, it  
says the Capacity is 465.44 Gb, 295.42 Gb available and 170.02 Gb used.

Currently the anti-clock has been turning for 10 minutes, yet still 0  
back-up.

Has anyone any idea of what I have done wrong, and how I can rectify  
the problem? TIA

Clive (in Spain)