CRTS are notoriously difficult and expensive to service. If it's still under warranty, then RMA it. If not, then you may be better off buying a new monitor. Any fix would be expensive and might not offer the results you're looking for.
I just wanted to know for interest sake whether technicians can alter the position of the electron guns or whether the guns are sealed inside the tube and cannot be altered in any way
....technically it is possible but they would need specialist gear. The average PC technician would not have this equipment or the knowledge to use it. Returning the monitor to the manufacturer for service is the only realistic option, although if it's not under warranty you should really be looking at buying a new monitor.
I don't but one our technicians used to work for Gateway and has particular knowledge of this area of technology. I can build a good PC but I can't claim to be a guru in any one area.