diff --git a/OSMapApi.js b/OSMapApi.js
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index 0000000..2ba6ee3
--- /dev/null
+++ b/OSMapApi.js
@@ -0,0 +1,115 @@
+/**
+ * os-leaflet ; A [Leafletjs](http://leafletjs.com/) TileLayer to display Ordnance Survey
+ *       data in your Leaflet map via the OS OpenSpace web map service.
+ *
+ * https://github.com/rob-murray/os-leaflet
+ */
+(function (root, factory) {
+  // UMD for  Node, AMD or browser globals
+  if (typeof define === 'function' && define.amd) {
+    // AMD. Register as an anonymous module.
+    define(['leaflet', 'proj4leaflet'], factory);
+  } else if (typeof exports === 'object') {
+    // Node & CommonJS-like environments.
+    var L = require('leaflet'); // eslint-disable-line vars-on-top
+    require('proj4leaflet');
+
+    module.exports = factory(L);
+  } else {
+    // Browser globals
+    if (typeof window.L === 'undefined') {
+      throw new Error('Leaflet missing');
+    }
+    root.returnExports = factory(root.L);
+  }
+}(this, function (L) {
+  /* This is our namespace for OSMapApi on Leaflet js */
+  L.OSMapApi = L.OSMapApi || {};
+  L.OSMapApi.CRS = L.extend(
+    new L.Proj.CRS(
+      'EPSG:27700',
+      '+proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000 +ellps=airy +datum=OSGB36 +units=m +no_defs', 
+      {
+        resolutions: [896, 448, 224, 112, 56, 28, 14, 7, 3.5, 1.75],
+        bounds: L.bounds([[0, 0], [938560, 1376256]]),
+        origin: [-238375.0, 0], 
+        maxRows: [5, 11, 23, 47, 95, 191, 383, 767, 1535, 3071],
+      }
+    ), {
+      distance: function (a, b) {
+        return L.CRS.Earth.distance(a, b);
+      }
+    }
+  );
+
+  /**
+   * A custom Layer for Ordnance Survey Maps API service.
+   * Note: An API key is needed, see OS website for details
+   *
+   */
+  L.OSMapApi.TileLayer = L.TileLayer.WMS.extend({
+
+    initialize: function (apiKey, options) { // (String, Object)
+      if (!apiKey) {
+        throw new Error('OSMapApi layer requires an API Key parameter to function.');
+      }
+
+      options = L.extend({
+        maxZoom: 9,
+        minZoom: 0,
+        zoomOffset: 0,
+        tileSize: 256,
+      }, options);
+
+      L.TileLayer.WMS.prototype.initialize.call(this,
+        'https://api2.ordnancesurvey.co.uk/mapping_api/v1/service/wmts', 
+        options);
+
+      this.wmsParams = {
+        key: apiKey,
+        service: 'WMTS',
+        request: 'GetTile',
+        format: 'image/png',
+        version: '1.0.0',
+        layer: 'Leisure 27700',
+        tileMatrixSet: 'EPSG:27700',
+      };
+    },
+
+    getAttribution: function () {
+      return '© Crown copyright and database rights ' +
+        new Date().getFullYear() +
+        ' Ordnance Survey. ';
+    },
+
+  /**
+   * Return a url for this tile.
+   * The tilePoint is measured from the top left corner with negative y values.
+   * The tileRow is measured from the bottom left corner with positive values.
+   * The number of map tile rows between the two origins varies with zoom level.
+   */
+    getTileUrl: function (tilePoint) { // (Point, Number) -> String
+      var OSZoom = tilePoint.z + this.options.zoomOffset
+      let maxRow = this._crs.options.maxRows[OSZoom];
+      this.wmsParams.tileRow = (maxRow + tilePoint.y + 1);
+      this.wmsParams.tileCol = tilePoint.x;
+      this.wmsParams.tileMatrix = 'EPSG:27700:' + (OSZoom);
+
+      return this._url + L.Util.getParamString(this.wmsParams); // eslint-disable-line no-underscore-dangle
+    }
+  });
+
+  /*
+   * Factory method to create a new OSMapApi tilelayer.
+   *
+   * @public
+   * @param {string} apiKey Your API key for OSMapApi.
+   * @param {object} options Any options to pass to the tilelayer.
+   * @return {L.TileLayer} TileLayer for Ordnance Survey Maps API service.
+   */
+  L.OSMapApi.tilelayer = function (apiKey, options) {
+    return new L.OSMapApi.TileLayer(apiKey, options);
+  };
+
+  return L.OSMapApi;
+}));
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