Speeding fines?

Hello all quick question on this, this morning I received a Notice of intended prosecution for doing 37 in a 30(mobile van). This I can except and will duly pay if I have to. The question I have is the (NIP) arrived today dated 1/4/14 for the offence dated 3/3/14 - Is the 14 day notice time still legally valid and with that they took a calender month to send it make it invalid or can I challenge based on time taken to send it?

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