Why does the light fitting flash/spark when microwave on.when ordinary fan oven on.

Kent, most microwaves will turn on the light when you open the door. As for the fan coming on, does it do that all of the time or only after you have used it for a period of time? If it does only after you have used it, you might want to check the back and the underneath of your microwave to see where the fan draws air into the unit and clean any lint built up on the vent holes. Hope this helps!

mine did the same thing took back apart cleaned put back togerther no fix bout new one

This not good, take warm soapy water and clean until spotless and test, if okay no further action required, if no change! check for any bare metal inside the interior, if everything checks out, the magnetron maybe the culprit. If it has a hood it will need to be replaced.

If there is spark please do not switch on unless you have confirmed the fault. It is possible that the Magnetron is leaking and sparking to the ground. As you have mentioned it was stored, it is quite possible that insect can get into the oven and create soft spots. When you switch on these will create a short and make the voltage jump and in the long run damage the magnetron.
So Remove cover after disconnecting , check for dirt/moisture in the cabin, if so wipe clean and dry, check the magnetron and area clean and use an jet air to clean off.
Now switch the microwave power on and use a cup of water to test.
If you notice any sparks shut off as the magnetron or the main HT capacitor can be faulty. Replace with same type. If not check the waveguides within the cabin for food particles that can burn and spark, check for rust that can corrode and spark when the ceramic paint has worn out.
If there is heating without any issues, you can use the microwave safely

Could be that water got in some ware ware it shorts to ground and the earth leakage are tripped. I had a similar problem with a microwave oven. You can leave it to dry for a day or two? I left the microwave in the sun for a few hours.

Yes, use the oven as though it is a fan oven because when the cavity gets to temperature the fan cuts in.
Hope that helps.

The flashing can be sparking of the magnetron please do not switch on unless you have confirmed the fault. It is possible that the Magnetron is leaking and sparking to the ground. If this was stored, it is quite possible that insect can get into the oven and create soft spots. When you switch on these will create a short and make the voltage jump and in the long run damage the magnetron.
So Remove cover after disconnecting , check for dirt/moisture in the cabin, if so wipe clean and dry, check the magnetron and area clean and use an jet air to clean off.
Now keeping clear switch on , use a cup of water inside to test.
If you notice any sparks shut off as the magnetron or the main HT capacitor can be faulty. replace with same type.
If there is heating without any issues , you can use the microwave safely

The fan that cools the magnetron also helps exhaust water vapor. Does the fan sound as strong as it used to? Is the vent at the back of the microwave clean? Sometimes lubing fan motor bearings with light machine oil will speed up the fan. You would have t remove the cover to gain access.

check exhast fan fittings and clean the bush..

The 'Clock, is Machanical just like an ordinary kitchen timer The reast of it is electronic, and at that age time for a new one.