The access manhole is just inside and to the left of the entrance to Victoria Park. The sewer runs along the line of Everest Rd. so if it does intercept the roundabout it would be only at the extreme Northern end, and may not intercept the roundabout at all: because I'm not local I can't check the lines. Also, at that point, the depth of the sewer is in the region of 21 metres so it's difficult to imagine what they might be doing which would get near it.
@ Excoriator: yes, it has been confirmed that the overflow tunnel started down Borough Rd. roughly at Parkend Rd., so nearly at Singleton Ave. MrG65 provided a lot of information but I haven't seen him posting recently. He had access to the original plans.
Very interesting. It seems that the interceptor at Parkend road not only takes the flow from the original Borough road sewer, but also what comes down Parkend Rd. This seems to be a substantial tunnel itself so it is probably what comes down Singleton Avenue.
I had assumed that this interception would be in the form of a weir, so that the old sewer would take the flow under normal circumstances and the excess would go over the weir into the new tunnel. But this seems not to have been the case. A large penstock implies the idea is to divert the lot into the new tunnel.
I wonder where the old tunnel ends up these days? It seems to be a substantial size, and from the lie of the land would pick up a lot of flow from the rest of birkenhead. I guess it would be Morpeth Dock, so I imagine it gets there via Argyle Street, probably joining the Great Culvert just before the treatment works.
Thanks very much for taking the trouble to dig out this info.