I want to set up a settlement at spectacle island, Preston wont send me there. I also have a few other workshops not unlocked and need to know how to set these up. Any ideas?

For most settlements you just have to clear the area of enemies and activate the workshop

Only disadvantage to doing that is that you miss out on the experience for doing the ~mission~ to set up a settlement at that location. Some places, the mission to establish the settlement is picked up AT the settlement (Greyditch, for example).

when I first went to the island, it was empty apart from the mirelurks etc. I have killed everything there, although it doesn't give me a cleared tag on the map. I really want to build there. I have found an ID tag for the mission, but don't know how to activate it. I'd rather do it legitimately, but preston only gives me the same handful of missions.

The thing is the 'mission' you miss out on is nothing more then what you would do if you ran across the place exploring and cleared of 'bad guys' I have come across at least 4 Settlements while exploring that were not MM missions that are now part of the greater MM community. I have cleared at least two others that as of now I do not have the resources to turn into Settlements but I will once I acquire enough. Before I am all done with the main mission every possible Settlement will fall under the MM protection.

My BIGGEST complaint is now that I have to manage all these Settlements, the "They need help" quests NEVER stop. They really need to get it under control, no more then one or two every couple of days.

got it sorted. I wasn't able to start the quest, but by activating the switch in the tug, then activating the one in the hut. I can now use the workbench.

You also know that when you do that the radio blasts the "Come to dinner" signal and draws a BUNCH of crabs you have to fight off. But of course you must know that, if you did it then the horde quickly followed, I was glad I took my power armor over there, I made short work of the entire bunch.

resources are not a problem on the island, just cleared it. There are thousands of materials there

It is a pain when there are so many settlements, but I eventually managed to get it under control for the most part. Here are the steps I've taken

1) Clear out settlements you don't plan to use anytime soon by consolidating to your choosen settlements. You can do this IF the population there are named 'Settler' are aren't kids or parents of a kid. Even if the settler can't be moved through the move command, you can move them by assigning them to a supply line and then waiting at the desitation for a few hours (use a bed or chair) and then reassigning the provision to a local job when they arrive. It helps to give them a unique enough outfit that sets them appart from any other provisioners you may have. Named families and settlements with a kid have to stay though.

2) Make sure remaining settlements have their needs met. Build defense up above food+water to reduce the chance of attack. I usually aim for 2x that number as it seems the chance goes down with greater defense... I can't really prove it though.

3) Never turn in Preston quests unless you want more. He will always want you to have at least one active quest. If you do the job (get past any timed portion... ie, do the rescue) but don't turn it in, you can hold off getting more quests from him.

4) Never listen to Minutement radio. It will constantly generate quests for you.

5) If you see a Settler not doing their normal job and sort of staring at you... or moving towards you... the Settler probably has a quest they're going to lay on you when you get close. I'm not sure if these can be avoided forever and you probably want them to get back to work eventually, but you can at least avoid them for a while.

I love the idea of having repeatable quests, but I wish it was done like the BoS or Railroad where you can choose to take them at your own pace... the Minutemen just lay it on thick and never do anything on their own. It becomes especially annoying with timed quests like rescue missions...particularly when it's the same Abernathy daughter getting kidnapped every time. Too bad we can't just gear up a few Minutemen with gauss rifles and power armor to act as her personal escort.