Am I right in this:
Month 1: 300 x 7% / 12.
Month 2: 600 x 7% / 12
Month 3: 900 x 7% / 12
etc. etc.??
Unfortunately the answer is no. Also, I'm not surprised by the branch staff being unable to help; I do IT work for retail banks and interest calculation is often seen as a black art.
The interest plus principal at the end of a period for a single deposit should be:
contribution * power(1 + annual_rate, months_in_deposit/12)
In your case, for the first contribution for example, this would be
300 * power(1.07, 11/12)
Note this assumes it is 11 months elapsed. Obvious this would be very tedius to do this for each of the contributions so that's why I'd always use Excel (or the OpenOffice equivalent, which is free). If you have Excel, the expression
=FV( power(1.07,1/12)-1, 11, -300 )
Gives you your answer (net of DIRT). Which in your case is 3,395 which means you've earned 95 interest. If DIRT has been charged then you should have earned 76 euro interest.
However, interest is more likely to be calculated daily rather than monthly which will affect the calculation but not to a huge extent. This also assumes the 7% is AER (annual equivalent rate), which it may not be. By law, loans have to be quoted using an APR while in theory they can quote any sort of figure for savings rates. This is probably the cause of the missing fiver. I'd ask them to specifiy the AER on the account.