bathroom waste pipes

[slipperymonster]

Currently my bath & sink pipes come outside then go thru' a flat roof/single brick wall 'store room' into an open 4" cast iron drainage pipe. I want to re-route outside to the same drain to avoid the damp/mould problems & get more space in the room. I was looking at joining sink(32mm) to bath waste (40mm) and then onto drain. My concern is it's 3.4 metres from bath outlet to going vertical to drain (which will be about a 2.5m drop). I can get a 1:40 angle.
Any reg's say I can't do this ?
Any reg's on how far a downpipe has to be from a window ?
Envisage any problems ?
Would it be advisable to go to 50mm diameter pipe ?

thanks