hey djmp,
As someone who has had lots of fun with medical, I can tell you that if you are prepared with all the relevant documentation for when they ask you about it you will be fine. If you're in for the Additional Testing, that means you've gone through the YOU session medical. If you raised this then and they didn't make a fuss it means it shouldn't be such a huge issue come Assessment Day.
Make sure to get all the relevant reports for any xrays or imaging performed. Make an appointment to see your specialist if you saw one for a few months away if your referral is valid so you can go and get a report if they ask for it. If they don't ask just cancel the appointment.
The reason everyone has headaches with medical is they go in expecting to be "good to go" and next minute they are unprepared and made Class 3 or 4. Depending on who you see be prepared to have to get all these details and get a letter from a specialist to avoid this categorisation. Getting a 3 or 4 isn't the end of the world however. You can still be classed fit for FSP while not Class 1.
Good luck with the additional testing and if you have any more questions fire away.