2004 Renault Megane Engine Logo

Background
1. Three weeks ago during bad weather (rain) the roads where blocked with traffic. Big delays, stop, go, stop, go. After 45 mins. I noticed the 'engine' logo +'service' was illuminated.
2. Into garage, on tester - they declared 'camshaft
centre' - Part due two days later.
But later that day the warning signs - vanished!
3. Car went to garage for part to be fitted. Came back OK. I did not question what work had been done -assumed it was sorted.
4. Three weeks later flashing 'engine' logo, then at times solid engine logo, before back to flashing again.
Engine reponse changing dependant on dashboard
signs.
5. Garage tester declares coil faulty - No 2!
Unusual I thought that tester would not be so specific -i.E declare a particular coil.
6. I've replaced the coils 1, 2, 3 - but number 4 apparently OK.
7. However the car appears to be OK - Until it gets up to temperature - then does not run as smooth unless on higher revs.

Car only due for service in another 9,500 miles. I've
only owned it for some five weeks.
Dealer it was bought from gave me a 12 month warantry but seems to be relying on a local Renault dealer for service support.

My previous Megane was fantastic, once I sorted out the coils. These replacement coils I had in stock.

Question:- the camshaft sensor appears to be a unit on top of the gear box. Though apparently just three weeks old - appears to be dirty! I suspect it was never changed earlier -though they charged 1.5 hours time. I'm not a mechanic but taking out two bolts and clipping in a cable switch - seems to be a
little job.