Hello Buster,
Ive just stumbled across the same problem with my mini mill and found it extreamly interesting reading your thread. After machining a 100 mm 'square' sheet of aluminium and checking it for square I found my mill was .2mm out of true and like yourself assumed the dovetails on the saddle are machined out of square to each other. can you please inform me as to the engineering company you used to have the saddle re machined as I live not too far from Bedford.
Thankyou
steve