You haven’t done too well up to now and you need to get a tax Investigation specialist on board as clearly from your post HMRC will prejudge ( probably correctly) that you have deliberately (and fraudulently) evaded Taxes.
You are also pushing up the penalty they will ultimately seek with your attempt to sidestep an ongoing investigation and not responding timeously to informal requests for information ultimately forcing them to initiate formal Sch 36 action.
If you had read the voluntary disclosure guidance you would have clearly seen that you cannot get the more beneficial terms of the voluntary disclosure campaigns once you’re caught with your trousers down and HMRC have issued their investigation opening letter.
Basically you know that matters are wrong in at least your let Property and depending what your other sources of income are ( e.g. if you were self employed or shareholder/director of a company ) you can expect their investigation to be thorough and outside the voluntary disclosure facilities. Depending on the sums and sources and periods involved where you have been guilty of tax evasion matters could be even more serious than you think and you might even have to consider looking for the protection of Code of Practice 9 ( see link www.wamstaxltd.com/codes_of_practice.html
In your present situation you need to take IMMEDIATE action to engage with an experienced tax investigation specialist ( not all competent tax advisers can deal with tax investigations to get the best results going) who will be able to act proactively to get the right way forward and not just react to HMRCs requests that will have to be responded to in time limits and lead to mistakes being made that could be costly when it comes to final settlement. There is nothing truer than every Investigation is one of a kind and you only get what you pay for. As regards timescale and costs how long is a piece of string.
With your past record of delaying and only telling part of the story you will have to change your spots as you will need to be wholly truthful with your adviser to ensure that you come out of this with the best possible outcome at the earliest possible date. Without wishing to overstate this I would explain that telling HMRC porkies or withholding relevant information are ways to get yourself moving out of the “civil settlement contract” route into the taxes criminal and potential prosecution category.