CO at 60 ppm is not excessively high.
Was that a service on your water heater and if so why did the engineer not correct it if he could.
First of all this could be how your parked up. Getting bad wind conditions on the flue or being to close to some type of obstruction.
The first thing to check is has the appliance got a good supply of air.
Make sure that the balancing flue air intake side is not blocked in any way. Could be caused by a number of things spider cobwebs leaves etc etc.
If that is OK some gas water heaters have an adjustment to regulate the amount of gas entering the appliance excess CO could be due to over gassing. If not adjustable a replacement gas valve will be needed.
But most important of all you must check that anyone working on your heater is gas safe registered and has the LPG category on their ID card.