A member of the public noticed, and alerted the local authority to the defect in the lining that had developed as a result of work to the road above the tunnel. (Navigation and towpath were then closed as of Friday afternoon...)

https://i.postimg.cc/Vv3S749S/Sydney-Gardens-number-two-tunnel-roof-issue-1600.jpg

 
We walked under there on the Thursday as they were working above and I wondered how deep they were going!
I had a look on Monday from the East end of the tunnel but there was nothing to see.
I wonder how long it'll take to repair?

 
Hopefully a bread and butter repair, albeit above water. But then... listed building, at least two agencies involved, negotiations to be had...

Off topic, from a quick exchange with a group of people working at Blisworth Tunnel in the late 1980s: they indicated that I think it was £27 per square metre was the going rate for brick lining repairs at that time. I have a faint recollection that didn't involve simple repointing but with brick lining, where do you stop...

 

The trench itself was shallow, though whatever was being excavated into was a bit concrete-chunky. It was difficult to make out exactly where the displaced roof blocks were in relation to the ground above. I had the impression that they weren't beneath the trench at all (which in any case hadn't reached the ground above the tunnel). They may have been below the road's west side pavement which has been resurfaced using tarmac, and probably using a plate compactor, the vibration from which may have exposed a couple of stone blocks where the mortar was failing.

It made me think of Devonshire Tunnel, which is lined with 'Shaped rubble' that may have gained a fair amount of additional cohesion courtesy of limescale among other things. Not, thankfully, that Devonshire is a streaming wet tunnel.

 

It made me think of the Battle of the Somme, frankly. 

CfN.