There has been a common fault with the 305's and 301's in as much as the power/data contact points on the back of the watch become tarnished from the sweat on your wrist when you use it.
This means that it not only won't charge correctly but also won't connect to the cradle properly when you want to download etc.
Have a look at the little round contact points on the back of the unit...are they a shiny gold colour, or slightly blackened? If they are blackened, rub them lightly with an eraser and they should clean up a bit. Then try charging it again.
If this all works then wearing a sweatband on your wrist under the garmin stops the problem recurring.
If all the above doesn't work, then you'll not be wearing it in Dublin I'm afraid, but get in contact with Garmin and they should be able to help you. I know there was a period when they were replacing Garmins quite frequently, which had a problem such as yours.
It's O.K now. I got the answer by googling "reset Garmin 305"
Basically hold reset and mode key together and then press the start key and it restarts. Also its keeps the history! Thanks for all your suggestions.