JJ, I don't know about the CAA Overseas Exams, but I suspect they use the same syllabus as the UK exams. If this is so, you have been given some doubtful advice
Speaking of the UK exams, Met Practical only requires you to decode METARs and TAFs, not to know Ops Procedures about selecting destination and diversion weather. Use the link given by cavortingcheetah to memorise the decodes.
As a direct answer to your question, in TAFS, FM followed by a 4 figure UTC time, which is hours and minutes, introduces a permanent change to the group(s) listed that will occur at the time given and will then persist until overtaken by another later change. BECMG followed by a 4 figure UTC time, which is two whole hours, introduces a permanent change just as FM does, but which may occur as early as the first time and will certainly have occured by the later time.
In METARS FM and BECMG only occur in the TREND section at the end, with slightly different rules about timegroups
Dick