Corsa 2003 Speedometer

Hi,

I own a Vauxhall Corsa 2003 (1.0 active).

After getting in my car last night I realised my speedo wasn't registering the speed until my car hit around 20-25 MPH (or at least that's what it felt like to me). So after getting in my car again today, it did the same thing, the speedo is sitting at 0MPH until my car reaches around 20MPH.

Any ideas to what's happening here?

Thanks in advance,
Jim.