Citroen C3 yellow engine light flashing, switch engine off and fan comes on. turn engine on engine light is on and steady and fan goes off?

I don't know what was done on your service , some " services " are just basic levels and visual checks . You should take it back to whoever did the work . Could be anything from a cooling fault to a coil pack just to name two possible causes . AA should have checked to see what fault codes had triggered the light .

Thanks
The AA got the car started and
It had two fault codes
P15A7, engine oil pressure too high
P0599, electrical thermostat signal high
Both cleared and enabled me to get it to the garage. On service he noticed the plugs were in poor condition and it was possible this was the cause of the non starting.
Now this, is it connected to the service or something altogether new? Only had the car 6 months and I want rid but who would buy it in this state?

Those two fault codes are serious and need addressed if they return .

I’ve had it back at the garage and they’ve checked the new plugs that were put in. Initially when putting them in they noticed oil in one of the plug sockets. They cleaned the socket then inserted the plugs. On inspection today they found more oil in the plug socket, they cleaned it again and reinserted the plug. They think the Rocker box gasket has gone and it’s leaking oil into the plugs, so it needs a replacing. I’m done with the car now and just want to part x for a newer car. Thanks for your reply’s.