Yes, there is a key on the exhaust pulley, but it allows about two-tooths of rotation. (The V6 engines have no key at all).
When I fitted my belt the first time, it all went well except that I forgot to tighten the pulleys before removing the camlocks and turning the engine over. And also, there had been the small matter of the white lines on the belt lining up with the exhaust pulley but not with the crank pulley.
The second and third times, I found that I could not tension the belt without the crankshaft moving.
Finally on the fourth time, I discovered that the crank pulley has a second mark (behind the balance belt pulley, so won't be as hard to find on the 1.6). I therefore lined up the white lines on both the exhaust cam pulley and the crank pulley. And then, I had no problem tensioning the belt. In other words, even when the pulley is slackened, it is still important to get the belt on exactly the right tooth of the exhaust cam pulley, because of the limited movement available for tensioning.
So I'd suggest to try again and also this time, make sure to line up the marks on the belt, to ensure you have the exhaust cam pulley on the right tooth
And as for knowing which white lines to use, you'll find that one white line is between two teeth (which is for the exhaust cam pulley), while the other white line(s) are in line with a tooth.