Corsa-B.uk - Vauxhall, Opel and Holden Corsa B Owners Club

I have a water leak from the offside rear of the engine, somewhere around the top of the gearbox, below the inlet manifold. losing about 500ml/week. I've had a good look underneath but can't find it - have also looked from the top but ideally need to remove the throttle body to see properly. Also can't see it running from anywhere (although it must be given the volumes involved).

I'm aware head gasket can be an issue and wondered if maybe it had an external leak (no other head gasket symptoms).

Are there any likely suspect places??