Further to my earlier posting I took the c5 to citroen to have it diagnosed.
The diagnostic results show s2000p code. The garage told us this is an
intermittent fault to the exhaust air injection. They said it would require
further investigation and would possibly need another 2 hours on the test
equipment at a cost of £162.15 to verify this.
What damage if any would be done to carry on driving or what can be done to
rectify the fault. The warning light is no longer on.