Many thanks for writing the DIY. I have had this problem for over a year and it's great to finally have this sorted. I didn't quite follow your DIY as my car is RHD and the vacuum hoses and access is different, but it gave me some good ideas to where my vacuum leak was. My garage had previously changed the pipes and check valve in an attempt to fix this, but it didn't help. However they had not changed rubber gasket, so trying to replace this was worth a shot. To be on the safe side I decided to change both the gasket and check valve.
As luck is the gasket is a lot more accessible on a RHD car and with a few long tools it's possible to replace it fairly easy. So I chose this option rather than going down the gasket maker route.
I have added a few pictures here that may help others see what the gasket and check valve looks like.
Brake check valve (RHD and it's quite different shape than the LHD) (Pulls straight out of gasket, other end can be disconnected quite easy by pushing the white tab in and pulling the pipes apart (remember to reduce the pressure by pumping the brake pedal). )
Gasket in place on brake booster (RHD)
Easy to fit new gasket by slipping you hand down the gap and then press the gasket into place. You may need some leverage to push the check valve hose back in again as it's pretty tight there. I used a long screw driver and used the side (not the end as this may do damage to the valve) to apply pressure and push it in.