Note that you will need to purchase the Custom Design upgrade in order to make CSS customizations.
To add an attribution to the footer credits, add this to the Appearance → Custom Design → CSS page in your blog dashboard and you can click the preview button to try it out before you buy the upgrade:
.credits:after {
float: left;
padding-top: 10px;
color: #959EA1;
content: " Produced & Promoted by Toby Elliott of 50 Phoebe Way, Oakhurst, Swindon, Wiltshire SN25 2JP";
}
Adjust the padding-top and color as necessary.