I fancy going marine (from tropicals) but I am somewhat clueless HELP :)

There are several threads on cycling--the stickied one at the top is fairly comprehensive, and will explain much of what you'll need to understand. One thing you will definitely need to support corals will be more light--there are a few that will work under what you have, but not many.

Otherwise--cycling SW is pretty similar to FW, though it's more common to use a few raw cocktail shrimp instead of bottled ammonia for a fishless setup. There is a great thread in the Marine archives that lists a variety of good reading material.

RobUK,

Start by reading this thread:
http://www.aquariacentral.com/forums/showthread.php?s=&threadid=11232

It addresses a great deal of what you need for a tank and how to cycle a marine tank pretty thoroughly. Once you've read that, feel free to ask more specific questions for your setup.

If you don't take any special steps, most tanks are safe to start adding invertebrate life to after 3 months. You can speed that up by adding liverock or live sand at the beginning of the cycle, but anything living on that rock or sand will die except the bacterias that control your ammonia, nitrites, and nitrates. If you have cured live rock and fresh livesand, you can almost skip the cycle altogether. It depends on what you start with.

If you want corals, you will need stronger lights for your tank. You will want at least two powerheads that move at least 280 gallons per hour for current. How much current depends on the corals you want to keep. You will probably also want a decent protien skimmer for that tank as well.