Citroen C5 rear ABS sensor

[crosser69]

Ive fitted a earlier C5 arm(2001) to a later C5(2007),all works fine but the abs light has come on,thought I had knackered the sensor but my mate put his machine on,he did have a technical word for it but basically the distance between the sensor and ring is too big,so basically it looks like I need a earlier sensor..Has anyone got one cheap I can buy to check? Or am I best changing my later hub onto the earlier arm??