I live in a top floor flat. The rear rooms which I use as bedroom/office
get really hot as the sun is beating on them all day long.

Ive been looking at small air con units. Ive been told that its no good
putting the vent out of an open window and this doesnt look good either.

I cant put a vent hole through the wall to the outside as the lease wont
allow it.

If you're renting the place personally I would just vent it out the window.
If you're after a more permanent solution get a vent slate fitted on the
roof and plug into that. As one of the other posters mentioned, most half
decent portable air-con units come with an attachment for sash-type windows,
however we've found it works just as well dangling the hose out the window.
These things kick out serious heat so I wouldn't be too keen on venting into
a sealed space. Ours also collects around 4L of water per day!

Andy