Tax people are likely to be aware of what accounts you have and capital held which includes the amount of interest you are paid.
But under normal circumstances they would not access your accounts for day to day financial transactions unless they had reason to, and then under most circumstancrs this would be through requests for the your bank statements with your co-operation and you providing them.
If you want to elaborate your concerns I may be able to give a more open confidential answer.
Its rare they would get the bank to disclose your affairs if bank statements were provided unless they had reason to think there were other undislcosed bank accounts.
I would advise they are probabaly ensuring you are making a full disclosure of all accounts before establishing whether there is a need to ask for bank statements.
Usually though when bank account details are asked for - its to establish whether a risk is established
i.e do they need to ask for more
I think what you need to ask yourself, are they liklely to find something that they should have been made aware of - then your real best shot is to come clean - disclosure is always looked upon more favourably.
However if Im barking up the wrong tree - then you have nothing to worry about, and it may well this is all they want from you at this time - but I will be honest with my expert experience - its likely they have reason to be asking for this info - as its not the norm.
If you feel you need support I would urge you either go to the Citizens Advise Bureau or seek a good accountant.