At certain times (and with certain stocks) the spread seems to vary pretty wildly. How is this calculated? and is there anything to look out for that can change the spread?

djbinlid - take a look at the demo of Level 2.

This shows the prices dealers will sell shares and prices they are buying. The difference is the spread! The easier it it for them to buy & sell, then the smaller the spread. Also if they want to encourage buying & selling they will sometimes improve the price slightly. The dealers also take into account of the availability of shares on the market, which also can effect the prices they quote. - Try and find out how Level 2 works, then it'll make more sense!