As things stand I am still not able to locate accurate advice about the point in time when savings interest actually becomes liable for tax -ie which tax year does a particular piece of interest fall into for tax purposes. There is some advise that interest which is rolled up into a fixed rate bond is not taxed until the year it matures because although paid to the account it is not "available" to the taxpayer. Any clarity anyone can shed on this particular issue would be very helpful as I have been over my records for the third time now and cannot reconcile my figures with those HMRC are using. An average taxpayer might have no problem decoding this; with the illnesses I suffer from the disability they cause me I am not an average tax payer. One fact remains, when banks deducted interest at source my tax affairs were in order - if anything I overpaid tax - when the responsibility transfers to HMRC my tax affairs are reduced to the nost horrible mess.
Any savings institution will be able to provide you with a yearly tax certificate confirming the amount of interest and tax deducted for that year, often these fixed term types have an ability to pay tax annually or at the end of the term - often the choice is yours when you sign up for these plans. It should be relatively obvious from the plan documentation though. Note your ability to draw the money from the pot is something that will not be relevant when deciding when interest has been credited or not
- the date the interest shows up on the statement and increases your balance is the relevant date for tax purposes - note this would not be the case if its a memo note showing interest accrued to date but not credited to the account.
As for hmrc using unreliable totals, upon request i would expect them to detail how their figures have been calculated - other than the first year i would expect figures in your coding to be complete estimates based on prior year totals - these estimates can be changed upon request for your better estimates or actual figures - hmrc should always be able to confirm whether figures are "estimates" or actual totals based on info provided by your bank - you may need to specifically ask them though that question to find out.
Note come the end of the year the onus is always on you to communicate the actual amount of interest received to hmrc - presuming there is a tax consequence involved.
When sorting this i would ensure actual interest totals for each year tax year are confirmed and agreed with hmrc before sorting out how the p800 calcs need to be revised.