The most common reason for your toilet to keep running is usually either the ball-valve washer has perished and is not shutting the flow of water off. Or the siphon washer is not sealing properly on the mechanism causing water to overflow inside the toilet pan.

It all depends on what central heating system you have it. If you have a combination boiler then it is most probably the diverter valve which determines which way the central heating water is sent round your system. If you have a boiler and cylinder then it will be either your 3 port or 2 port valve, which in the same way determines which way the central heating water is sent.

Most common cause of a loud boiler crashing and banging would be that the central heating system is blocked or heavily sludge's up causing poor circulation and preventing to water to flow through your boiler quick enough. The best way to solve this problem would be to powerflush the heating system with chemical and magnetic system filters until clear.

Locate your gas meter usually under the stairs or in a cupboard downstairs or outside in a gas meter box. Shut the lever valve until the lever is fully horizontal.

The fist thing to do if you have any sort or water leak is to locate your mains water stopcock and isolate. Once isolated contact a plumber asap.