mk2 clio buzzing from relays & no start

[dean.st]

I've had this before on my sons mk2 1.2 16v clio but it just cleared itself before i could find out what it was.

Tonight it's back. When you turn on the ignition (to get all dash lights on) the battery light goes dim and the relays under the bonnet buzz constantly. The battery is new and fully charged.

Any ideas what could cause this?