The recent cold weather, and a stiff cable release, conspired to cause the secondary release handle to break.
It is a pretty flimsy bit of plastic I have to say.
A trawl of ebay came up with a complete new catch assembly, and the new plastic handle and cable.
It wasn't too much of a fiddle to replace, and the bonnet now open very smoothly
That was pretty much what happened to me! The metal bonnet catch itself was pretty manky, so for a mere 14 quid I thought it well worth replacing at the same time