I don't have an internal schematic for the speedo, so can't say why that should happen, but sounds strange that is the only thing that seems to be affected ?  The seatbelt switches a hard ground to the speedo pin 20 (Yellow wire) on the 32 way connector The switch in the seatbelt catch goes via a 6 pin connector under the seat

I will investigate asap. Thanks Nige. 

Well Nige, my lady has decided she doesn't want me tinkering around with her Polo anymore and says she can live with the seat belt light on. I would have liked to have found the problem but it's her car and I have to abide by her wishes.  I would like to thank you for all your assistance and comments. If I ever do find the cause, I will certainly let you know. Again, many thanks. Tim.