just wondering what tests they do at this?
Precise answer depends on your age, and when your initial was done.
However, to me it sounds as if you are fairly young, and I'm reading this as being your first renewal after your initial at Gatwick last year.
In which case you will need ONLY the "routine renewal medical" - no ECG, no audio, no special visual tests, no blood tests (apart from finger prick for haemoglobin), no CXR, no EEG etc etc.
The routine renewal is 1. check for any changes in personal data (address, phone, employer etc) 2. check for any changes in medical history since last time 3. basic examination, ie height, weight, urine, vision, ophthalmoscopy, ENT area, BP, pulse, heart and lungs, feel abdomen, and reflexes.
There you go. Simple, innit ?