Fitted a 3L catch can down at the bottom of the engine bay (09 2.2) with all associated same diameter hoses. No reduction in bore at all. Spent maybe £80 for this mod.
The results - absolutely no difference. Still oily mess inside the intake pipes.

My question, if i vent the breather to atmosphere somewhere convenient and away from the passenger compartment, apart from some fumes, will this have any adverse effect on crankcase pressure and resulting oil leaks? Only difference i can think of is that there will be no vacuum applied to the crankcase. But in anycase, this cant be that much (vacuum) surely?

Don’t know if you read This Thread?

It may be that cheap catch-cans don’t make a huge difference, while a decent oil separator might? 2005 D3 2.7 Auto
Previously:
2010 MY FL2 TD4e GS - Now gone at 199,500 miles, about 135,000 of them mine.
‘93 Defender 110 200TDi CSW
‘87 Defender 90 4 cyl Petrol
‘83 110 CSW V8 - best ever!
Range Rover 2-door V8 (not sure of year - 4-speed box and vacuum diff switch)
Series III SWB Diesel