hi gigster,
One of the reasons the diodes failed is because they are only rated for 1 amp.
If you have a 12Vrms 5amp transformer, you should have at least 5amp diodes, I would goto 10amp, if possible.
Connect a 5amp slow blow fuse in the output from the charger to the battery. [say, in +V lead]
The charger should also be mains powered from a fused supply, 3A fuse should be OK.
I would also ground the -V supply from the charger to the mains ground, just in case the charger transformer develops a fault.
As chargers are sometimes used outdoors, it would be advisable to use a RCCB in the mains supply line.
EricG
What type of battery are you going charge??