Thank you all for the thoughts and advice.
The insurance cover includes legal, but apparently this is to defend any action against policyholder.
If the wall collapses, and is deemed mine, any cover will be reduced because it was not maintained!!
Regarding which came first, our deeds are only for the plot of land, shows the boundaries but no details of the building.
The plot shows proposed road widening which may mean that nothing had been done, but stops development closer to the road for 4 houses. In 1937 the pavement may not have been much more than dirt, judging by some old photographs of the area.
The council are clear that they did not build, and will not adopt, the wall as it was probably built for the benefit of the development.
All boundary walls and fences are deemed to be the responsibility of both ourselves and adjacent landowners.
I have asked the council what the building regulations for constructing a wall to support the highway were in 1937 and, as the wall did not meet the specs (engineer confidant that the amount of rotation clearly shows it was inadequate), what they suggest as it should have been inspected and passed as suitable.
The engineer is suggesting gabions 1M square across the 8M, this will reduce my useable land, by more than a metre which is in front of windows. But may be the way to go as it does nor require deep excavation which may find cables and pipes with all that that entails. Have re-attached the proposal.
Retaining Wall Strengthening Proposals.pdf