No its not mandatory. You are required to EITHER hold your completed apprenticeship papers as a Diesel Motor Mechanic (Please note these qualifications could fall under Diesel Fitter qualifications) and 3 years of paid work experience pre or post qualification working as a Diesel mechanic. So, if you are from the UK and hold an NVQ3 as a Diesel Motor Mechanic or Diesel Fitter, you would only require the 3 years.
If you have NO NVQ3 then you are required to have a minimum of 5 years paid work experience completing diesel motor mechanic tasks. It is common for clients to be assessed without holding any formal training or qualification so we use your 5 years of paid work experience as proof.