Here is the Scottish government page confirming that Scots over 75 or immunosuppressed should be getting a second booster. This is to happen 24 weeks at least after the previous booster. I had my booster / vaccine 4 (after a 3rd primary before, because I’m severely immunosuppressed) just after Christmas. So I expect my 5th vaccine in mid June. With probably a 6th vaccine - 3rd booster - to follow, probably 6 months later.
gov.scot/news/second-booste...
The second boosters were recommended by JCVI and should be rolled out for these patients throughout the UK.