JSON Error on checkout line1 column 1

Hello, this is the first time i am requesting help on an issue, usually i can figure them out but this one has me lost. Ive started receiving the following JSON error at checkout:

SyntaxError: JSON.parse: unexpected character at line 1 column 1 of the JSON data

Upon inspection using Firefox Webinspect It appears the following data is being posted

<h1>8336</h1>{“result”:”success”,”redirect”:”http:\/\/localhost\/index.php\/checkout\/order-received\/8336?key=wc_order_565370c66de0e”}

so the “<h1>8336</h1>” seems to be causing the issue but I cant figure out what is attaching that data to the JSON query. All I know the 8336 is the order number at checkout. Ive disabled all non essential plugins but still no luck. Any help is very much appreciated!