1) I did a view source of your page. There's a .css file: http://porty.org.uk/tmp/jsn_epic_pro_css_37c96c9d5f25dc63ce13c447eef076b4.css
Think this is a auto-generated css file.
2) To shift the left margin of the map, you need to change line 27 of the css file in (1):
Change the padding from, say 1.2em to 0.2em.
You will see that the left margin of the google map will become 0.2em.
3) The top margin of the google map is caused by the following:
Which in turn uses line262 of the css file in (1):
Try changing the above from height: 1em; to height: 0em;
I'm not sure how the css file in (1) is generated. In any case, the above shows you which are the 2 css tag that you can change to shift the top and left padding/margin of the map.
You need to do a bit of testing to see changing the css file will affect other parts of your joomla page.
Hope this helps.