Complex sentence: The Air Ministry were reluctant to award Westland contracts due to Petters inexperience, but his reputation as a successful designer was strengthened when testing demonstrated the automatic slats on the P.V 7 were effective and reliable.
Split into simpler sentences: The Air Ministry were reluctant to award Westland's contracts due to Petters inexperience. But his reputation as a successful designer was strengthened when testinfg of the PV7 demonstrated the automatic slats were effective and reliable.