The wastage from an over the sink heater would be less than the hot water wasted by cooling in a long pipe run from an immersion heater.
Off peak rates are generally about 1/3 of the cost of on peak. so assuming both the immersion and the above sink heaters were about the same rating (3KW) you could run a over the sink heater for about 25min a day for the same cost as your immersion heater.
Given the fact you'd have to pay for the over the sink heaters and install them even if it did save a little per day it would probably take years to break even.