Yes it has to be within 14 days unless you were stopped at the time in which case it can be many months, mind you for 6mph you'd probably get an awareness course rather than a fine & points.
Nevertheless it's outside the statutory time limit so write back and say so. Go to
www.pepipoo.com for advice and a sample letter.
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Ian
Current: 2021 G21 330i M-Sport
Previous: 2018 A6 Avant S-Line MMI+, 2014 F31 320d M-Sport, 2013 F10 520d M-Sport, 2011 F10 530d M-Sport, 2008 320i M-Sport Coupe, 2002 325i, 2001 318i valvetronic, 1998 318i, 1996 525i, 1990 Porsche 944S2