When you want to add CSS inline to a tag you use the style attribute of the tag.
As TB was saying this is not the optimal way of using CSS tho.
<p style='your css rules here>text </p>
ALSO NOTE: FLOATED ELEMENTS must be placed FIRST in the source code. The way you have it the image will float.. but bellow all the text in the paragraph ( maybe that's what you intended, tho ).
<p><img style='float:right' src="GSR GIF 100.gif" width="100" height="100" alt="water leak repairs Gas Safe Registger logo"/> Emergency waterleak repairs in southeast London is one of the key services I deliver, seven days a week if need be. A lot of people think this is something that
will only happen in winter during long periods of severe frost. I can assure you that is not at all the case, and have included a list of some of the most frequently
found reasons for a problem of this kind to pop up. Some of these are utterly obvious, or at least will be when you think about it; others are not. And when it
happens and you can not control it, this simple, clean water that we all take for granted can cause an enormous amount of grief. </p>