I’ve had another go with the saw - popped the table off and found that the two front corner allen bolts are missing, as is one of the hex bolts holding the saw assembly plate to the table.
I’ll order some replacements. In the meantime, I replaced the table with the bolts loose and with only one roll pin fitted, so it could rotate. With the blade raised and set at exactly 90deg, I set a combi square in the mitre slot such that the rule just brushed a tooth at the front. I marked this tooth and rotated it to the back and re-checked. By tapping the table clock or anticlock I got the blade square to the mitre slot.
But as I tightened the table bolts, the thing would skew off again, and not in a predictable direction. I tried for hours, tightening the bolts in different orders and trying to compensate, but no joy. It’s ended up a hair looser at the back of the blade. I used it to rip some 1/2” thick, 5” wide panels from a rough block (in a double pass) so it’s not too bad.
When the new bolts come, I’ll do it all again, but should I put some washers on the bolts, to help stop them skewing as they tighten, or perhaps try to ream out the second roll pin hole and fit the pin before tightening?