Hi Shirley, post lunch, post Sunday Politics and after Caroline finished what she was doing on the Internet.
I am going to answer your other questions and issues indirectly based on my experience.
1) Many years ago, I had a discussion with my accountant in the UK about some income and he said Peter if you do not declare it, you will probably get away with it and in truth if you didn't you can easily afford the fine but don't do it as the IR (as it was in those days) will make your life a misery for many years into the future.
2) When I first came to France I went to see the Impots to discuss how I declared my income. I had a few pounds of interest from Abbey National and the man looked at me, more or less called me a bloody fool and crossed it out. Last year I met with a minister in the last Government at the Treasury and recounted this anecdote and he laughed and said the HMRC would do that here (in the UK)
I do know that there is an increasing flow of information about everyone who has income in the UK being transferred by the HMRC to the Impots.
That means that in all probability the Impots know your income to the penny but I also guess that "Shirley" does not rank highly in their search for tax evaders.
But in not declaring your income fully and/or correctly you are breaking the law and thus open to a future claim and fines by the Impots.
I am not qualified to tell you what to do as that is your decision.
Going back to your domestic situation, obviously I do not know the whole story and as I say I am not an expert but it seems to me that if the Impots do NOT know you are living alone and thus have only "1 part" rather than "2 parts" that is very material to your tax liability.
Perhaps I misunderstand but you must get advice on this matter from a tax expert, preferably one with knowledge of the UK system as well. There are quite a few who read these blogs so perhaps they may be willing to help you.