Balanced flue through slates, leaking

I would have a look at the metal annular piece that is attached to the flue and overhangs the sleeve on the lead slate. Especially the joint where it fits round the chimney. This usually has some sort of clamp and sealant. Needs to be watertight. Quite often weathering, the sealant going hard and expansion etc causes it to crack and leak in heavy rain.

In message , Tim W writes

I'd be looking at the bottom of the flue area, maybe wehre it meets the weather slate at the back, or of of the connections above. Say where the top gray bit is fixed to the lower grey bit (rivetted?)

Does it always leak, or only in some wind directions? I have a leak from that only happens under certain conditions, - though never quite worked out what, but it's needs to be heavy rain and windy.

I stopped water ingress by patching a small hole in the membrane under the slates (made by a piece of timber pushing against it ) but I never did work out how it came in (the bit of roof abuts a small bit of vertical wall going up above the roof, so it's probably related)