Simon, after trying a jig-saw, then a band-saw (both set on super-slow), I still managed to crack a side window. Peter Ransom (DRB) put me on the right track - a diamond blade on an angle grinder. It worked a treat - just 4-5 very light, quick runs & you end up with a super-clean cut.
For finishing off the edges, I used a belt sander clamped upside-down to the bench (a proper belt linisher would have been better). After each test fit, I marked the amount to be removed from the edge with a red fine-tipped pen, then ran the piece lightly over the belt until the red line had gone. Test fit, re-mark, & do it again - lightly.
For drilling the holes, I found a drill-press the best as you can control the pressure accurately. Unless you have a very low-angle drill bit, it will tend to "bite" - a hand-held drill will just run thru & probably crack the piece, whereas a drill-press will allow you to go a lttle way in, back out to let things cool, go back in a little, etc, etc.
After I had done all my holes & countersunk them, I heard that this was not a smart idea - c/s heads try to split the persex when tightened up ! Apparently the best approach is to have straight-thru holes & to use pan-head bolts/screws with thin nylon washers.
Makes sense when I think about it - I'll know how to do it properly next time !!
Kind Regards,
Peter D.