a: trying to apply a £2400 payment to my accounts that i have already explained to them is accounted for in the subsequent year's accounts. (which would mean they
are trying to count it twice which is ridiculous )
If hmrc are being picky they are within their rights to adjust entries based on the correct accounting principles rather than where you wanted to declare it .
What was your rationale for declaring that income in the subsequent year - if it was a deposit for goods / services provided for in the next accounting year i would expect them to back down once you provide evidence the money was "earned" in the following year. Even if they are proved right in the adjustment make it clear that they accept its a timing issue and that they will consent to that subsequent year being revised the other way - hopefully that following year is not out of time in that regard.
b: have asked for my personal bank account statements from 6 April 2011 to 5 April 2014 to support property income computations.
I would second what warmstax says - unless they have found anything wrong with the year of enquiry that would suggest there was issues in previous years i would stick to the year under enquiry only - get them to confirm that this is acceptable - if thats not acceptable to them ask them specifically what legislation allows them to look outside the relevant year if nothing has been proven wrong with the year under enquiry.