Giro Slip Barcode & numbers below the line

On the Giro Slip Barcode I have 24 digit number under the barcode (yours might be different). Have tried to find a complete breakdown of how to interperate this but have been unable to.

Digits 1-7 could be a bank sort code of some description but when trying the 1st 6 digits on sortcodes.co.uk it comes back 'not found' although digits 2 through 7 do come back valid, sortcodes.co.uk will only take 6 digits for sort code & 8 for account number, if your account is 7 or less then insert leading '0's 1st.

Digits 8 through 13 are the same as the bank sort code stated earlier in my utlitiy bill where there is information I could pay out of my account for online banking (no definatly not).

Digits 14 through 23 are the same as the 'Reference (Account Number)'

Digit 24 is a single digit, highly likely meaning there is an 11 digit bank account number (the 10 digit 'Reference (Account Number)' + digit 24) with a 6 digit sort code (Digits 8 through 13), being 1 of the private accounts we are not suppose to know about.

I have been unable to find after some searching as to whether an 11 digit UK bank account number can be verified - exists, this is something I could do with help on.
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At the bottom of the tear off slip the is a line & text above it 'do not write or mark below this line'.

I have 4 groups of numbers followed by an 'X'.

Group 1 is obviuosly the 10 digit 'Reference (Account Number)'.
Group 2 if read right to left is the same as the 7 digit 'Credit account number', granted there are extra characters to the left but cannot find thier meaning.
Group 3 if read right to left is the same as the amount in the thick black box but with a slight twist. Lets say the cash value was £852.77 I would have in mine '000852771' reading right to left there is an obvious extra digit the '1', meaning not exactly known, it could be part of the 'cash value'.
Group 4 has 74 & that 'X', I did find out & post in 1 previous thread that the '74   X' meant 'with added value' - joint giro but as of 31/12/2018 joint giro has gone for 'new customers', so all we now have (I do) is added value.

LS