JavaScript Code will not run

I have a Javascript calculator I made a few years back which works well in a HTML page but when I add it to a WordPress page, it will not run.
I have a smaller script in the same page which does work so I know I’m linking to .js file correctly. I have tried different methods of calling the Javascript code, all work with the smaller script, none with the bigger unless Im in a standard stand-alone HTML file.
Any help would be very much appriciated.

HTML Code

<table>
<tbody>
<tr>
<td><select id="mlen" name="mlen"><option value="2">2 Metres</option><option value="3">3 Metres</option><option value="4">4 Metres</option><option value="6">6 Metres</option><option value="8">8 Metres</option><option value="10">10 Metres</option><option value="12">12 Metres</option><option value="14">14 Metres</option><option value="16">16 Metres</option><option value="18">18 Metres</option></select></td>
<td>by</td>
<td><select id="mwid" name="mwid"><option value="2">2 Metres</option><option value="3">3 Metres</option><option value="4">4 Metres</option><option value="6">6 Metres</option><option value="8">8 Metres</option><option value="9">9 Metres</option><option value="10">10 Metres</option><option value="12">12 Metres</option><option value="14">14 Metres</option><option value="16">16 Metres</option><option value="18">18 Metres</option></select></td>
</tr>
</tbody>
</table>

<hr />

<h3>Marquee Extra's</h3>
<table>
<tbody>
<tr>
<td>Image</td>
<td>Main Marquee</td>
<td></td>
<td></td>
<td>£<span id="totmar">0</span></td>
</tr>
<tr>
<td>Image</td>
<td>Lighting</td>
<td></td>
<td><input id="mlig" type="checkbox" name="mlig" /></td>
<td>£<span id="totlig">0</span></td>
</tr>
<tr>
<td>Image</td>
<td>Flooring</td>
<td></td>
<td><input id="mflo" type="checkbox" name="mflo" /></td>
<td>£<span id="totflo">0</span></td>
</tr>
<tr>
<td>Image</td>
<td>Heaters</td>
<td></td>
<td><input id="mhea" type="text" name="mhea" size="1" value="0" /></td>
<td>£<span id="tothea">0</span></td>
</tr>
<tr>
<td>Image</td>
<td>Tables</td>
<td></td>
<td><input id="mtab" type="text" name="mtab" size="1" value="0" /></td>
<td>£<span id="tottab">0</span></td>
</tr>
<tr>
<td>Image</td>
<td>Chairs</td>
<td></td>
<td><input id="mcha" type="text" name="mcha" size="1" value="0" /></td>
<td>£<span id="totcha">0</span></td>
</tr>
</tbody>
</table>

<hr />
<input type="button" onClick="mtot(mtot)" value="Calculate">
<strong>Your estimate total is</strong>
<strong>£<span id="mtot">0</span></strong>

.js Code

var pri = []; // Price structure.
          pri['mar'] = [10.00,7.50,6.50]; // Marquee prices.
          pri['lig'] = [1.00,0.75,0.50]; // Lighting prices.
          pri['flo'] = [3.00,2.50,2.00]; // Flooring prices.

      var priTab = 8.00; // Table price.
      var priCha = 2.00; // Chair price.
      var priHea = 50.00; // Heater price.

      function est() {
        var rat;
        var tot = 0;
        var squ = parseInt(el('mlen').options[el('mlen').selectedIndex].value) * parseInt(el('mwid').options[el('mwid').selectedIndex].value);

        if(squ > 100) rat = 2;
        else if(squ > 29) rat = 1;
        else rat = 0;

        var cMar = squ * pri['mar'][rat];
        el('totmar').innerHTML = cMar;
        tot += cMar;

        var tLig = el('totlig');
        if(el('mlig').checked == true) {
          var cLig = squ * pri['lig'][rat];
          tLig.innerHTML = cLig;
          tot += cLig;
        }
        else tLig.innerHTML = 0;

        var tFlo = el('totflo');
        if(el('mflo').checked == true) {
          var cFlo = squ * pri['flo'][rat];
          tFlo.innerHTML = cFlo;
          tot += cFlo;
        }
        else tFlo.innerHTML = 0;

        var Tab = parseInt(el('mtab').value);
        var tTab = el('tottab');
        if(Tab > 0) {
          var cTab = Tab * priTab;
          tTab.innerHTML = cTab;
          tot += cTab;
        }
        else tTab.innerHTML = 0;

        var Cha = parseInt(el('mcha').value);
        var tCha = el('totcha');
        if(Cha > 0) {
          var cCha = Cha * priCha;
          tCha.innerHTML = cCha;
          tot += cCha;
        }
        else tCha.innerHTML = 0;

// Heater Settings

        var Hea = parseInt(el('mhea').value);
        var tHea = el('tothea');
        if(Hea > 0) {
          var cHea = Hea * priHea;
          tHea.innerHTML = cHea;
          tot += cHea;
        }
        else tHea.innerHTML = 0;

// Total

        el('mtot').innerHTML = tot;
      }

      function el('ele') {
        return document.getElementById(ele);
      }

      window.onload = function() {
        el('mlen').onchange = est;
        el('mwid').onchange = est;
        el('mlig').onclick = est;
        el('mflo').onclick = est;
        el('mhea').onkeyup = est;
        el('mtab').onkeyup = est;
        el('mcha').onkeyup = est;
        est();
      }