There were some hidden characters which were causing an issue. Just copy and paste below piece (you may not find differences but do it).
var markers = [
{
coords:{lat:53.470912,lng:-2.241365},
iconImage:'https://developers.google.com/maps/documentation/javascript/examples/full/images/beachflag.png',
content:'<h1>John Dalton East Carpark</h1>'
},
{
coords:{lat:53.471740,lng:-2.240842},
content:'<h1>John Dalton East Tower</h1>'
},
{
coords:{lat:53.472499,lng:-2.240830},
content:'<h1>John Dalton Foods</h1>'
},
{
coords:{lat:53.470493,lng:-2.238513},
content:'<h1>AllSaints Park</h1>'
},
{
coords:{lat:53.442371,lng:-2.201845},
content:'<h1>My Home</h1>'
}];