1997 Fiat Punto 1.7 TD 70 Diesel won't start - key code?

My punto wouldn't start recently, I thought it may be the battery (which is getting on). So I have charged it quite a few times and the car still won't start.

Have checked with multi-meter and standing the battery is 12.5V and when trying to turn over it drops to 10V so still within reasonable values.

I don't have an immobiliser and certainly no keypad to enter a code but the 'code' light is staying lit on my dashboard. This has come on and off at random intervals whilst i've had the car, but recently it has been staying on longer and sometimes whole journeys. I'm not sure if this is somehow stopping the engine starting.

I have seen online people talking about red master keys. Well I have 2 smaller blue keys and a larger reddish-brown key. have tried this in ignition and same issue remains.

any help greatly appreciated!