We have just noticed that our toilet cistern is filling up really slowly. Also the pan when you flush is empying slowly and the water level rises to 2/3 of the depth of the pan.
Any thoughts?

Sounds like you have two problems.
Assuming that what you call a cistern we call a tank and what you call a pan we call a bowl.
You have a partially blocked toilet or sewer pipe. You can probably clear this with a toilet plunger, if not you will have to have it snaked or "rodded" out to clear blockage.
Next you probably have a defective washer or diaphram in the ballcock or fill valve in the tank. What we call a ballcock will have a ball on the end of a long rod that acts like a float to turn the water off when the tank is full. The fill valve will have float (usually black) around a shaft that the water comes in on. Both do the same thing and have washers in them that can be replaced. Replacing the washer in the fill valve is real easy. You simply turn the water off, reach in under the float, lift it up and grasp the shaft. Then turn the cap on top of the shaft 1/4 turn counterclockwise. Remove and replace the washer and replace the cap. Replacing the diaphram in the ballcock is basically the same. You remove the cap of the by removing the screws holding it in place and replace the diaphram.

I have a toilet drain pipe that is leaking and inaccessible without cutting a hole into my ceiling. Is there a way to insert a new pipe within the old, past the leak?