Defining grid-names: Setting up a 1m2 grid over a 100x100 meter area

I'm no programmer and there is probably a more elegant/efficient/'Pythonic' way of doing this than the code shown below. However, as a novice learning the ropes of PyQGIS, this was a fun task to undertake. I've tested this in QGIS version 2.18, if you need to convert it to work with the QGIS 3 API, hopefully someone on here can help you with that.

I define a function to transform the coordinates of each cell and create nested loops to populate a list of character strings.

Enter the coordinates of the south west corner in the 'coords' variable and all the points will be rotated around this point.

You add a new script to QGIS through toolbox > scripts > create new script, a full tutorial can be found here.

from qgis.core import *
from PyQt4.QtCore import QVariant
import string, math

#Parameters to set up
epsg = '27700' # Enter epsg code here - British National Grid shown as example ** Use a projected crs that uses metres for units
coords = [111111.11, 111111.11] # Enter the coordinates of the south west corner of the grid, this point will remain unchanged and all other points will be rotated around this
angleRadians = math.radians(-5.5) # Enter the rotation of the grid here in degrees. Enter a +ive value for CW rotation or -ive value for CCW rotation
maxCols = 100 # Maxiumum number of columns in the grid

#Create new layer and add a field called "ref"
lyrSetupStr = 'Polygon?crs=epsg:{}'.format(epsg)
lyrOut = QgsVectorLayer(lyrSetupStr, 'grid', 'memory')
lyrOut.startEditing()
newField = QgsField("ref", QVariant.String)
lyrOut.dataProvider().addAttributes([newField])
lyrOut.updateFields()

#function to transform points using trig
def func_rotate(x, y, degree):

    #this part works out transformation relative to start position 
    xOut = x * math.cos(angleRadians) - y * math.sin(degree)
    yOut = x * math.sin(angleRadians) + y * math.cos(degree)

    #return the transformation plus the coordinate of the starting grid to get the absolute position
    return xOut + coords[0], yOut + coords[1]

#create a list of lowercase ascii characters
alphaList = list(string.ascii_lowercase)

#create list to hold column references
rowIdList = []

#loop through first set of a-j rows
for i in range(10):

    #loop through second set of a-j rows and append to list
    for j in range(10):
        rowId = [alphaList[i], alphaList[j]]
        rowIdList.append(rowId)


#loop through rows
for rowNum in range(maxCols):
    #get the row reference from the list
    rowId = rowIdList[maxCols - rowNum - 1]

    #loop through columns
    for colNum in range(maxCols): 

        #call the transformation function for each of the 4 points of the cell square
        pt1 = func_rotate(maxCols - rowNum - 1, maxCols - colNum - 1, angleRadians)
        pt2 = func_rotate(maxCols - rowNum , maxCols - colNum - 1, angleRadians)
        pt3 = func_rotate(maxCols - rowNum , maxCols - colNum, angleRadians)
        pt4 = func_rotate(maxCols - rowNum -1, maxCols - colNum , angleRadians)

        #turn the returned coordinates into Well Known Text
        wkt = 'POLYGON (({} {}, {} {}, {} {}, {} {}))'.format(str(pt1[0]), str(pt1[1]), str(pt2[0]), str(pt2[1]), str(pt3[0]), str(pt3[1]), str(pt4[0]), str(pt4[1]))

        #Create a new feature, set the geometry 
        f = QgsFeature()
        geo = QgsGeometry.fromWkt(wkt)
        f.setGeometry(geo) 

        #add the ref field to the new feature and populate
        newFields = QgsFields()
        newFields.append(newField)
        f.setFields(newFields)
        attOut = '{}A{}{}'.format(str(maxCols - rowNum), str(rowIdList[colNum][0]), str(rowIdList[colNum][1]))
        f['ref'] = attOut

        #add the feature to the new layer
        lyrOut.dataProvider().addFeatures([f])

#add the new layer to the map
QgsMapLayerRegistry.instance().addMapLayer(lyrOut)
lyrOut.commitChanges()