I have a very large pimple/cyst on the back of my leg that has grown quite rapidly over the past 3 days, the inflamed area is more than 15cm across with a very small head, what is my best course of action?

Get examined: In the short term apply moist heat over the area & it may help it mature & start to drain. However, one this size needs to be opened & drained surgically (possibly in office) or it is unlikely to heal. Temporary improvement may come with some drainage but it will tent to re-accumulate until it is properly opened & the core/rhine removed.