Oh I see the saltwater community is getting acquainted with Oscaroo. He caused quite a fuss in the cichlid sections when he joined so hopefully he knows now to read some books instead of just have no knowledge and asking every question that comes to his head.
Oscaroo, I've kept reef and fish only tanks about 5 years ago. I read extensively on the subject for around a year and now am setting up another saltwater aquarium and I still have questions and research to do. You can't expect to just walk into saltwater from fresh and know what youre doing. It's a completely different system.
You want to start off by cycling the tank, put all of the water and sand, live rocks in there with the correct salinity. then you cant just drop food into the water once a day or so to increase the ammonia build up. this will give you the Ammonia cycle, something which you should do some research on.
It will take you anywhere from 2-5 weeks to mature the tank. it's different for everyone. My guess for that tank would be about 2 weeks, but you need to keep checking the water with tests to know when the tank is fully cycled. If you can get your hands on a few buckets of mature water from an established marine tank then you can knock the time down to a week maybe less. same goes for a handfull of established gravel.
ps. you might want to tell your mum that reef lights will make quite a sharp increase in your electric bill. I know when I sold off my reef aquarium the bill went down by a few hundred pounds (which would be even more in dollars)