Probably the best type of the water butt could be constructed from the downpipe from your gutter system.
Method:
Make this by cutting the pipework and run it straight down into the actual water butt. There is a thing called a rain diverter. This bit of kit attaches to the downpipe and through a 90 degree flexi pipe will as said divert the rain water into the butt. It is possibly included in the water butt kit that you have bought. Now run some overflow pipework from the top of the butt to the base of your downpipe. Water butts come with instructions and all the parts that you need. Barking Plumbers can fit water butts.
Tools:
Tape measure.
Hole cutter.
Drill.
Plumber’s merchants should have choices of pipework, so you could take off a connector (should be push-fit), then take it with you. You could measure the external diameter and try and search for one. Square pipework is easy to measure, however, for round ones you will need calipers or string and a rule. Wrap the string around the pipe and mark where the two ends meet up. Take this measurement, and then, divide the measurement by 3.14 (PI). A Barking Plumber can make this calculation. This number gives you the external diameter of the pipework. You need to confirm that the inlet and overflow pipework are around the same size. If they aren’t the butt will probably overflow if there is a heavy downpour. The kits resolve this as they include an inlet that only collects small volumes of water. It is usual to want to trap all the water, particularly if you have a large water butt and the water is coming from small area. To do this the flow must be adjusted.
Note: When the diverter is used it is possible that the overflow is not required as long as the diverter is fitted lower than the top of your water butt as it will stop filling as soon as it is full.