I used to pay 750 baht per month into the SSO when I worked in Thailand but that was a number of years ago. I'm now over 50 and retired. Am I able to, and is there any benefit to doing so, paying the 750 baht voluntarily? If I do, will my medical cover return in six months time?

Not in your case. You have to start voluntary payments within 6 months of finishing your employment in Thailand. Voluntary payments are 432 baht per month.

 

Your only option would be to gain employment in Thailand for a minimum of 12 months and pay Social Fund during this period, then leave your employment and then volunteer for payments within 6 months of your job ending.