Both roads are too small to sustain heavy usage and Manor Mount and St. David's Roads are way too small for two way traffic.
Is Manor Mount narrower than Ewelme Road, Woodcombe Crescent, Benson Road, or Devonshire Road? Ewelme and Benson look just as steep as Manor Mount, so if steepness is a criteria they should be 'No Entry' too. They certainly have a greater claim on the basis of being 100% residential unlike Manor Mount and Waldenshaw which provide access to: car park, supermarket, hair salon, cafe, restaurant ....
Perhaps Wood Vale could reasonable be asked to share more of the traffic - many of the dwellings are set back quite a way from the road, and much of the road is alongside the cemetery.