Battery help

The battery showing fully charged does not necessarily mean that the battery is ok. I would check the voltage while starting the engine, then you would know if the battery really is ok.

All the advice above is good and correct, particularly the age related comments.

May I add some basic science, Specific Gravity, this relates to temperatue and state of charge and is the only true indicator of battery health.

When I lived in Jersey, I had a "no starter motor/use the crank handle" temp, and a "no crank handle/no alternator light" temp as well, thats when I got a lift to work. When things returned to normal, above 4 degrees C, it started ok! 

May I suggest a web page called "Battery University", there are many pages on the science, I won't point you to a specific one, you fill find one to meet your needs.
 

You could have a problem with the starter motor solenoid. Every time you use the s/motor the heavy current burns the contacts and over time this burning will destroy the copper contacts. It's another possible reason for your problem but can be cured with a re-build using new contacts.

Roger

Turning over slowly indicates an electrical problem of some description, either the battery, cables, earth strap solenoid has a problem and cannot supply the correct current or your starter motor is faulty.

Put a voltmeter on the battery and then try to crank the motor over, does the voltage drop a lot? If not then the battery might be okay, but you can't get enough current to the motor or the motor is dying.

If the voltage drops a lot your battery has some type of problem which is limiting the current, you can get a get meter which measures cranking current (Ancel BA101, etc), but they are about £45, so cheaper to take it to a garage. This could be any number of things as others have already suggested.

If the voltage is okay, then something else is limiting the current, are the battery terminals clean and tight? Is the earth strap clean and secured correctly?

After that it will be wiring, solenoid or starter motor, these might not be easy to check if you don't have the equipment, so it might be a trip to a garage.

Thanks for the replies, I got a voltmeter and with the engine off the reading was 12.1 which I believe it should be 12.6 at least and on starting the engine it drops down to 8.5 then goes up to 14.7 so I think this means the alternator is ok and I should be looking at hopefully just replacing the battery? 

Thanks 

12.1V is fairly flat, but it does depend on temperature, if it is very cold it might be okay. However dropping to 8.5v is a sure sign of a dead battery.

If you charge the battery and it is still like that then the battery has died and you need a new one.

If it goes up to 14.7V when charging that indicates that the alternator is okay, but again the battery is dead, it should only reach 14.7 when fully charged.