Do I need to do anything with my current pension when I leave? Or if I leave it will it just continue to sit there until I’m old enough to use it?
If you leave it, it will just sit there and continue to grow (hopefully) until you're old enough to use it. So you don't *need* to do anything. If you move jobs reasonably often, it can be easy to end up with lots of small pension pots like this and be paying fees on all of them, those fees will reduce the performance of the investment, so it can make sense to consolidate all of them in one place.
So, you can also transfer the money that's in it into another pension. The standard advice round here would be to move it into a self-invested personal pension (SIPP) and invest it yourself, into something like Vanguard Lifestrategy (VLS80 etc.) or Vanguard World Index (VWRL).
Obviously, you will want to join the workplace pension in your new job too.