Can anyone tell me if this is the speed sensor?

[white exec]

Very much doubt that is the speed sensor, but always possible.
Speed sensors, when they fail, tend to intermittently drop out, causing the speedo to flutter, badly under-read, or drop to zero. Can also cause a cruise control to stop functioning.

You could try a replacement - not too expensive, but suggest a genuine part from Citroen, rather than a cheap copy.

[Paul-R]

Has the distance reading been accurate? From what you're saying I would think it has been.

?

[xantia_v6]

The speed sensor sensor produces a fixed number of pulses per revolution, set by the mechanical design. It either works or it doesn't.

[white exec]

Not quite - the quality of the output signal can deteriorate, either through mechanical wear, or debris (despite being sealed units). Symptoms as described above.