arcpy - Selecting files with same first 5 characters in filename for merging?

I have a number of polygon feature classes in a geodatabase dataset. The file names are as below:

e.g. IHO1a_xxx-xxxx_ALB,IHO1b_xxx-xxxx_MBES_10m, IHO1b_xxx-xxxx_MBES_2m

I would like to get a python script which will select all files starting with exactly the same first 14 characters in the filename and merge them.

import os,arcpy

# Define location of .gdb

arcpy.env.workspace = "D:\Users\Documents\H-11_Documents for   Reports_Paul\Test_Model_Builder\ICP_Intermediate.gdb\Deliveryxx_Clipped_Po        lygons" 

#Make workspace a variable
workspace = arcpy.env.workspace

List = []

for dirpath, dirnames, filenames in arcpy.da.Walk(workspace, datatype="FeatureClass", type = "Polygon"):
    for filename in filenames:
        if filename.startswith(filename[0:13]):

            List.append(os.path.join(dirpath, filename))
print List
arcpy.Merge_management([filename,filename], 'D:\Users\cadetpn\Documents\H-11_Documents for Reports_Paul\Test_Model_Builder\Temp.gdb\Temp_Data\merge%n%')

##To keep console window open
raw_input("Press enter to exit...")