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Thanks ed.
The lug was definitely sqeezed, in the end i could put the 27.2 seatpost in while widening the lug with the help of a screwdriver.
So if my guess with the 531st seattube is right, Tony Oliver is also right - guess the Roberts is silver brazed, so another 531st with 27.2 diameter here.
The Roberts rides really really nice btw.. Will post some pictures if it has been converted from its ugly trekking-ness to drop bars...
Thanks everyone...cheers.
Simple; it does not go in.