You Will Need a Gas Safe Engineer If The Work That You Need Carried Out Includes Any Gas Work. Ask The Gas Engineer To See There Gas Safe Card and On The Reverse Side There Will Be a List of Gas Appliances In which They Are Qualified To Work On.
If You Just Require Plumbing Work, You Do Not Need A Gas Safe Engineer. Many Gas Safe Engineers Also Carry Out Plumbing work.
But Not All Plumber Do Gas Work.
Hope This Helps
The Posh Plumbers