Focus Speedometer/odometer intermittent fault - any suggestions

Ford Focus Ghia (Sep '02). Lately the speedo fails to register until I've driven (typically)5-10 miles, then continues normally. While it is stuck at zero mph, the tripmeter and odometer do not advance. At some point they both revert to showing dashes instead of digits. When the speedo kicks in again, the tripmeter and odometer continue from the values they had when the speedometer dial was at zero. As far as I can determine, when the speedo is working it is accurate. Nothing else on the instrument panel is affected.

Can anyone offer a suggestion as to what might cause the behaviour described above?

I'd like to approach my friendly local garage with a 'rescue plan' that is "focussed".

Thanks for your attention.

David