an ordinary, vented cylinder like yours doesn't generally need servicing. It the unvented ones, because they run at high pressure and are fitted with safety devices that need to be kept in good order.
Your green one will be copper. I don't remember when the newer blue ones came out. Somewhere between ten and twenty years.
It does look like it is leaking at a pipe connector. An experienced plumber may be able to fix that. It doesn't need a gasman. The copper is quite thin and easily damaged by heavy-handed spannering. A replacement is not especially expensive.
Before you decide, measure the water flow into your house. Fill a bucket at the kitchen cold tap; time it' calculate litres per minute. Do the same at your garden tap and utility room cold tap of you have them.
Have a look at the incoming pipe from the watermain where it meets your stopcock. Depending on age of house it will be lead, iron, copper, black plastic or blue plastic. Measure its outside diameter. It will probably be 15mm, 20mm, 22mm, 25mm, or 32mm.
If it passes under wooden floors and flowerbeds it would be easier to upgrade than concrete floors and drives.