These questions are generally quite difficult to answer without seeing them on a working site. Luckily this one's very straightforward. The text is forced to the center by this statement towards the end of your stylesheet.css file
.centeredContent, TH, #cartEmptyText, #cartBoxGVButton, #cartBoxEmpty, #cartBoxVoucherBalance, #navCatTabsWrapper, #navEZPageNextPrev, #bannerOne, #bannerTwo, #bannerThree, #bannerFour, #bannerFive, #bannerSix, #siteinfoLegal, #siteinfoCredits, #siteinfoStatus, #siteinfoIP, .center, .cartRemoveItemDisplay, .cartQuantityUpdate, .cartQuantity, .cartTotalsDisplay, #cartBoxGVBalance, .leftBoxHeading, .centerBoxHeading,.rightBoxHeading, .productListing-data, .accountQuantityDisplay, .ratingRow, LABEL#textAreaReviews, #productMainImage, #reviewsInfoDefaultProductImage, #productReviewsDefaultProductImage, #reviewWriteMainImage, .centerBoxContents, .specialsListBoxContents, .categoryListBoxContents, .additionalImages, .centerBoxContentsSpecials, .centerBoxContentsAlsoPurch, .centerBoxContentsFeatured, .centerBoxContentsNew, .gvBal, .attribImg {
text-align: center;
}Basically everything in that list is centered including elements with the class that I have highlighted in red. If you remove the one in red (including the comma afterwards) the text in your screenshot will again default to the left.
As to your second question, I'm afraid that I'm not sure specifically what you consider to be "additional text". Most of what's there looks to me like text that you have added.