The short circuit current rating (Isc) and the Max power current (Imp)of the PV panel is higher than the Inverter ratings.
The inverter engineer's have said that the inverter will be find for these currents even though the inverter label says different as well as the data sheet for the inverter.
Should I ask for something in writing from them stating that the inverter will be ok or insist on the inverter been upgraded?
You're the one who'll be signing the CoC, and who will therefore be open to come-back from the end-user.
So if you've got what amounts to a CD, all you have to do is follow it.