After speaking to Machine7 (http://www.machine7.co.uk), I was informed that the maximum stud pattern which would just fit on their blank brake discs (both front and rear) is 139.7mm.
My first choice wheel is available with a 6 stud pattern (and 6 spoke design) in a 6*139.7 fitment, so I went ahead and ordered the wheels today from raceways (http://www.raceways.co.uk) 
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To show my lack of design skills, here’s an image of my old car with these wheels on to give a rough idea of how they will look.
