Do you have to uncoil the entire length of the lead of an electrical cassette reel extention lead before use?

It depends on what job you are doing, a small job for a short time, no problem. A bigger job or a long job then uncoil the lot. A coil of wire acts like a magnet when current is passed through it and it builds up a magnetic field that heats up. I have seen a coil with all the wire welded together.

im a builder ..from experience i would always say uncoil it all ...start using tools with a loading and the cable will get too hot to handle if left rolled

Depends on the load you are taking - a heavy load could do damage, so err on the side of safety if you're not sure, and unwind all of it.

typically, no. it is safe just to uncoil what you need.

The recommendation is that you should completely unwind the lead before use. That is because the coiled cable represents a reactive load in series with whatever you plug in. If you use the extension with a small resistive load - such as a table lamp, there are no problems. However if you use it with a large reactive load - like a fan heater, the coiled cable will get hot and could catch fire. If you don't understand the difference between a small resistive load and a large reactive load, always unwind the cable fully.

You can just uncoil what you need, no problem.