I'm probably reading the question in an incorrect way, but just in case...
28 square metres is not the same as 28 metres squared - pug has given you the answer for 28 square metres, but if you really do have 28 metres squared it's 784 square metres, i.e. 28m long x 28m wide.
That would need almost 4000 slabs...
also - your 28 square metres - if it's 7x4 you'll need 144 slabs as you won't want to put bits of slabs in to make up the area. 7 metres long would take 15 slabs plus a piece of another, and 4 wide would take just over 8.
Hiya, well I just worked out roughly 25 m2 for an L shaped patio im quoting and came up with 120 of the 450mm2 flags, and just came here to check if I was off, but that Wickerman is good!
But being a Druid I would usually say never trust them Wicker people nurf nurf