Aarggh, I bought the wrong bolts - I chose the M8 x 20mm from
here and they're far to fat to fit the hole in the trailing arm. I mixed up socket size with thread size - an 8mm socket will undo the originals, so I bought M8 size.
Originals: 8mm socket fits the head but thread diameter is 5.9mm and thread length is 12mm (overall length 18mm).
The ones I bought: 13mm socket fits the head but thread size is 7.9mm and thread length is 20mm (overall length 27mm).
So everything I said earlier is wrong.