It parses OK for me when I escape the forward slash with a reverse slash:
if (preg_match("/<a href=\"www.mysites.co.uk\"><b><span style=\"text-decoration: none\"><font size=\"2\">Mysite - <font color=\"#000000\">short description here <\/font>/i",$data)) {
PS: you can make your life a bit simpler by using single quotes around the whole regex string, since you are not interpolating any variables within it. Then the only thing you'd have to escape is that slash:
if (preg_match('/<a href="www.mysites.co.uk"><b><span style="text-decoration: none"><font size="2">Mysite - <font color="#000000">short description here <\/font>/i', $data)) {