Hi James, I've had a quick look and overall I like the clean layout, but feel it could do with some colour in places to liven it up a bit, such as the "FIND OUT HOW MUTABLE LABS CAN HELP YOU" banner at the bottom. This is a call to action, so make it stand out. Similarly, the testimonial/s could do with some colour or excitement to draw attention to them. The person's name on a coloured background perhaps? You could even look into separating the sections with some colour, just to help break it up a bit.
I'd also mix up the typography a bit. Currently it's all very "samey" on the index page. Same weight, same style, same face. This leaves the whole page looking a little dull and I found myself struggling to read through the copy. I'd keep it short and snappy. If you feel you'd like to tell people more about yourself, stick it on an "About" page (these are generally considered to be the second place visitors to your site will go to, so it's worth having one to help sell yourself).
Finally, I'd remove the "coming soon" portfolio piece. If you don't have something to show, leave it out until you do. Never pad out your portfolio with filler content for the sake of it. Having a small selection of quality pieces is better than loads of average projects.