The windows have all recently been serviced as this has been an ongoing issue, but it seems the last attempt was also unsuccessful at correctly sealing the windows - hence the whistling in the wind. Makes me wonder why I ever bothered upgrading to double glazing when the wind is just able to sidestep the opening pane and sneak round the side and past the seal. These are A&L windows and after not being able to seal them on 5 service attempts, I wouldn't recommend them to anyone.
What with the constant whistling and what feels like the entire roof lifting and falling at times (and making considerable banging noises in the process - see around 3:00-4:00mins in the video) getting any sleep last night was not a possibility - I was more concerned about the roof coming off. Not to mention the front door still whistles like a kettle due to the large gap underneath it.
How this home ever complied to the 6* energy rating the paperwork states I'll never know. Perhaps I should get an independent tester in and see how it fares.