kar mba
The encroachment of the build may have been an established agreement with the previous owners with regard building on the property line where a 'party' garden wall has been replaced by the wall of the extension.
This does not in any way exonerate the trespass of the gutter / felt overhang which would need to be treated differently if you were to future build against this.
If you proceed with the purchase and in the process you solicitor overcomes the problem, by changing the deeds to reflect the property line, then you should have no issue into the future. If as mathepac suggests, you end up not buying, it won't be your concern.
If the problem can be dealt with and the record corrected, this should not be a future problem.
As for building against the wall, it wouldn't be advisable directly on the hollow block, but nothing is impossible there are ways of dealing with this.