3 bar would be fine for an unvented system. You can't go far wrong with a megaflo. When purchasing the cylinder don't be duped into buying a bigger one than you need. I am presuming that in your flat you only have one bathroom and possibly an ensuite? If you are only intending to have two showers or baths on the go at any one time you won't be needing a massive cylinder. MB
Its also worth adding that when you are doing your pressure test at the washing machine tap get someone to flush your toilet at the same time so you can get a true reading when the system is being used ie theres water being drawn off somewhere ie the toilet.
Ok, so they said no I can't have the external waste pipe running outisde the flat. Can they stop me from doing this? is there any way to make an internal adjustment so the overflow pipe can use the internal drainage system?
You can vent the pipe out through the wall then bend it back so it vents down the wall. You should contact your local building control officer and ask if thats a viable option. As far as connecting into existing waste pipes -you shouldnt do it because if the unvented cylinder starts discharging water it will be in the high 90 degree mark and will deform your plastic waste pipes.