I have a leak at the bottom of the cistern where the nut joins to the cistern. The leak is coming from the bottom of the cistern, dripping on to the nut and running down. I have rubber woshers on both sides, in an out the cistern. I have screwed as much as I can. Help. Pleace

The blue pipe with the elbow? Zip tie it to the overflow pipe, put a brick on it etc.

The flush control panel normally comes off. You can then hopefully see inside. You won't be able to see much though as the cistern will be in the way...

Places to look for leaks are:
Soil pipe to pan
Cistern to flush pipe
Flush pipe to pan
Cistern overflow.

Could be one of the carb floats is stuck or the rollover caused it to come off its mounting position. As fuel enters the carb, the float rises with the fuel level until it reaches a position that shuts off any more fuel entering the float chamber and prevents an overflow. Fuel level drops, float drops and fuel flows again. Basically like the cistern in a toilet.

Fixing a leaky toilet is actually rather simple, requiring few tools and just a little time and effort. In looking at Caroma toilets on Google, they appear to be just a standard type of unit, and should present few if any problems for yourself or any handyman of modest skill.

If it is leaking around the base of the toilet - where the toilet sits on the floor, the repair item is a simple wax seal. Here is a tutorial to effect this repair: Replace Toilet Wax Ring. The wax ring is very inexpensive, usually $5 to $10 USD.

If the water is leaking from between the tank (cistern) and the toilet bowl, the job is even easier, as it does not require lifting and moving the entire toilet. A good tutorial for that repair is here: How to Replace Tank to Bowl Gasket . The gaskets, new nuts, bolts and rubber seals come in a kit and also cost only $5 to $10 USD.

I am sorry to hear your Plumber insisted you buy an entire new toilet, but do remember he is in the business of making money!

replace the whole control unit
There may be 2 problems
the small washer that shuts off the water flow when the level is reached may be swollen and not sealing properly
the section at the bottom has cracked at a thread level
so by replacing the unit you will eliminate both problems
to remove the unit, turn off the water tap, flush the system, holding the button down until all of the water has gone
remove the inlet pipe at the bottom of the cistern
undo the retaining nut under the cistern and lift the control unit out
there will be water in the bottom of the cistern so use old towels on the floor to soak up the water that will run out

Are you sure it is leaking from a spot "Originating" at the bottom? Many times this is what is seen, but the water is actually traveling down the side.

If at the bottom, you either have a cracked tank, or the drain cap is loose, missing the cap gasket or it is on "Cross Threaded"?

But... most likely from experience, it leaking at the valve clamp.

there is an adjustment screw ontop of the fill valve assembly. Tighten this down and the water level should stop at a lower level unless the internal mechanism has crusted up.

Assuming the seal washer is inside the cistern and there are no cracks in the cistern itself you could apply a silicon sealant to the seal washer inside the cistern.