If you are a night worker the employer must offer you a free health assessment, however you do not have to accept this. Confirmation from the Government website here:
You are correct that compensatory rest is not there to cover meetings outside of normal working hours - it is mainly there if you have not been able to take your usual breaks due to working shifts, working extra unplanned hours, if your work requires continuous provision of care, etc.
Night workers cannot work more than an average of 8 hours over a 24-hour period. This average is calculated over a 17-week period (unless a longer period was agreed by you and the employer). So that means you could potentially work more than 8 hours on numerous occasions, as long as the average over this calculation period does not go above 8h over a 24h period.
Finally, there is nothing in law which states how long the investigatory process should take, it can take as long as necessary for the employer to gather the relevant evidence.
I trust this has answered your query. Please take a second to leave a positive rating by selecting 3, 4 or 5 stars above - this is an important part of our process and recognises the time I have spent assisting you. If you still need me to clarify anything else, please reply on here and I will assist as best as I can. Thank you