If you go to this link and enter 1000 in savings and 42% - I would have thought the top up was 420 if you are taxed at 42% - what am I missing here?? Where does the 724 come from??
This is the way for people who get pension conts deducted from salary roll.
To calculate the "gross" amount saved for high rate taxpayer take the amount of salary saved (i.e. €1,000) and divide by 58% (i.e. 0.58) to get the gross amount saved (i.e 1,000/0.58 = 1,724) then deduct the amount of salary conceded (1,000) to get the relief (= 1,724 - 1,000).