Paying Back by L.P. Gibbs
Alan Randall, a retired Soho enforcer, had carried out executions for a former Detective Inspector on evil people who had evaded justice, then was himself betrayed and shot. Amazingly, he survived the assassination attempt and set out to seek revenge.
This led him on to an unexpected path, bringing him into conflict with merciless foreign gangsters.
They didn't know Randall, though.
When his family and way of life is threatened, he dishes out his own tried and tested form of retribution with devastating consequences.
To fully understand and appreciate this novel, you are advised to first read (or re-read) “The Last Payback”, as this is the sequel.