I have some insight here as a large part of my former job was spent in recruitment and hence interviewing.
Firstly, I would not bother sending a thank you of any kind - employment law is quite strict in the uk and the best candidate at the interview should get the job, regardless of whether they are nice / polite or not. In fact, sending something may be interpreted as canvassing, which could disqualify you. ( a stretch I know, but always a risk!)
Secondly, I would not be afraid to ring - chance are you will get someone in HR who can enlighten you, not the manager. And they will be well used to these calls - I know I was!
Thirdly, it may just simply be a delay. Lots of interviews, the paperwork with that, HR staff being busy etc. although it is major event in your life, for those processing this it is just another day at the office and it may have dropped priority to something else.
Also, to answer previous points, the organisation will be obliged to interview all who meet criteria, and if a job has limited criteria then that will be a lot of interviews!
Lastly, please do not get disheartened!! I really hope you get this job, but if you didn't it doesn't mean you did not do well at interview. It's all about who is best, and though you may have been good, someone else might have been better (hope not though!!) I have often interviewed a really good candidate only to have someone better walk in the door. And that's just life
!
Keep going, there is a job out there for you x