Mark,
NIE numbers - you can't do anything without this and you have to attend the police station yourself to get it as far as I am aware for I/D purposes. Best way is through Polaris, we did ours the day before we completed and it was very easy with the Polaris guide.
Habitation Certificate - if you are not prepared to complete without this then forget completing in August. We completed on our Penthouse in December without the HC and there is still no sign of it yet. This is not a problem to me as we did not have a mortgage but many banks will insist upon it if you do. If you do need it then get Polaris to move now and confirm that it has been issued or your completion date will be a wasted journey.
Utilities - no sign of connection for us in block 11 of penthouses - I am certainly not pushing as the water and electricity is free under the builders supply - the air con is getting a good bashing at the moment!
Snagging - do it yourself would be my advice if you have an eye for detail. Take a torch with you as there will be no lighting in your apartment and checking for tile cracks in a bathroom wth no windows is impssible as we found out.
I was over at CDA until this Thursday last and the ISLA looks great and you will be delighted with your property and the effort in attending to complete and snag all in one visit to Spain is all part of the excitement for what is a major commitment in most people's lives.
If you need furter advice drop me a PM. Our completion was hectic but we managed to avoid all sorts of problems.
Good luck. Jonathan Faukner