I'm no guru on reading the swell forecasts, I noticed with the swell period at the moment on Thursday for example it is quite a high swell period, then it drops down considerable for a short period of time then back up to the original period quite rapidly. What does that mean? Is it 2 swells mixed together or something?

Two swells of a similar size, with one being short period localised wind swell. Have a look at a forecast site that breaks down individual swells and you see it straight away.