Another general note -- as a tutor, I'd like some assurance that I will get paid for answering the question. This may lead to less questions, but significantly more answers. This obviously introduces quite a bit of complexity for the poster, but I think its necessary. You could potentially start people with a few pounds in their account that you finance to get things rolling (or, give people a few pounds when they sign up and add a credit card).
Next -- visual design -- I was impressed with your design, there are some nit-picky things to tweak, but overall it is visually appealing. -In safari, the sign in/register links at the top have the descender part of the g gets cut off (probably because the containing div is too short). -The font color in the "Post My Question", "View All Questions", "Answer Question/Make Bid", etc is a bit difficult to read. This one nags at me quite a bit because it makes the buttons look pixelated or something -- i'd fix this up right away. -Really like the visual representation of description of service -> ask a question call to action. looks great/leads the eye the way you want it to!
As others mentioned -- the about us could use some copy. Don't want to spend too much time on it, but it adds a lot of credibility (or rather, it takes a lot a way to see 1 line of text there).
I have to run here in a second, but one last thing -- you might want to think about requiring signup AFTER someone has answered the question (as you do with the asking of a question). Once someone has spent time answering a question they are more likely to sign up.
Wish I had some more time, but the site looks good, keep up the great work!
Good luck!