It is a bit more economical to heat if fully, then use it until it is mostly gone, rather than to top it up every time you fill a sink.
This is because it is inefficient to keep heating up the boiler and pipes.
Megaflo and other unvented cylinders are usually quite big, for example 200 litres or so. A bath uses about 100 litres, including some cold.
Set the HW timer to heat the cylinder about an hour before you will be having baths and showers. If you have so many that you need more hot water than it holds, leave it running during bath time, and about half an hour after you have finished, and it will keep topped up. It will then, in most households, provide more than enough hot water until bathtime comes round again and the HW timer comes back on.
If you have a circulating pump to a distant bathroom, the hot water pipe will act as a long thin radiator so it must be thoroughly insulated.
If you are having mutiple baths and showers one after the other, leave the HW on and the boiler will heat it about as fast as you use it.
When the cylinder is fully hot, its thermostat will stop demanding heat from the boiler so it will not use any more gas until hot water has been used and the HW timer is on.