Hi forum, just closing out this post for future readers.
The upload throughput limit issue seems to now be resolved - see this other post for details (it needed the moderators to resolve and I am very grateful to them)
The download sync cap issue could not be resolved as the community moderators found that Openreach (probably quite sensibly) do not offer the 40Mbps+ tier on this line. Max predicted speeds reported from the line checker are 41.4Mbps so it would be a small increment over the 40Mbps currently achieved, so no great problem. Just interesting that it is not actually possible to achieve the estimated rate.
Thanks everyone for the help, especially @NeilO who has done exactly what he promised and chased this issue to the end.