How can I fix fuse (F5) blowing on my Rover 25 car?

I cannot operate the central locking on my Rover 25 X registered 1600 model. If I do the fuse blows and the engine is immobilised. The only way round this is to leave my car unlocked which isn't safe. I can't afford to take it to a car electronics garage just to find out what's wrong as I'm a retired lady on state pension. Can anyone help please ?