It will cost you a small fortune im afraid. Probably around £500 for the tank glass, £500 for a strong cabinet, £200 for a decent sized sump, £800 for the live rock, £500 for decent lights (probably more), £600-£1500 for a decent skimmer, £200-400 for powerheads etc, £200 for pipework and connectors, £200-600 for return pump, £200 for a decent RO unit. Then theres the stock, substrate, any other filtration methods you may want, heater, chiller, and other items you may want to consider such as calcium reactors, UV sterilisers, phosphate reactors etc etc etc...
Also consumables and running costs such as salt, filter media, food, additives, DI resin, replacement bulbs/tubes, test kits and the big one electricity.
That is all based on brand new equipment, obviously you can get some stuff second hand. If you join a forum such as ultimate reef, you could have a look at some of the large/supersized tank threads and ask the tank owner for a breakdown of costs. Im just estimating, but id say its pretty close to actual costs.
Edit - I hope its a ground floor flat btw! If not you need to check with a structural engineer as to whether your floor will hold the weight (may want to do this anyway even on a ground floor flat) you might end up having to re-enforce the floor.
Edit 2 - If you can afford it though, definately go for it. You wont regret it