Location of NR maintenance work

[SEPS]

I live near the railway and am informed by NR when they are undertaking maintenance work. Generally the location is given as x chains and y feet from London.
The latest letter gives the position as MLN1 13m 0512y.
13 m from where?

 

[Dibbo4025]

From Paddington - MLN1 is the egineers' line reference for the

 

[High Dyke]

A search here: http://www.railwaycodes.org.uk/elrs/elrm.shtm

MLN Main Line (Paddington to Penzance via Bath) see separate portions 
    MLN1 Paddington to North Road Junction 0.00 - 246.08 Paddington via Box   
    MLN2 Cornwall Loop Junction to St Germans 247.28 - 256.38 Paddington via Box and Plymouth Millbay (reverse)   
    MLN3 St Germans to mileage change point east of Chacewater 256.40 - 305.65 Paddington via Box and Plymouth Millbay (reverse)   
    MLN4 Mileage change point east of Chacewater to Penzance 305.67 - 326.50 Paddington via Box and Plymouth Millbay (reverse)   
    MLN5 Plymouth North Road to Millbay station 246.08 - 246.64 Paddington 246.64 (Millbay station) - 247.22 doubled
    MLN6 Bristol Temple Meads Old Station 118.16 - 118.39 Paddington   
    MLN7 Plymouth North Road west side of triangle 247.22 - 247.42 Paddington via Box and Plymouth Millbay (reverse)

Based on the information the OP has provided that would be 13 miles from Paddington, so therefore in the vicinity of West Drayton.