For a minute, to answer the question 'what boiler for the thermal store' you'll need to understand what you expect to take from the store, in order to match a suitable boiler to supply that demand.
So for example, if your property heat losses are similar to that of the boiler output, but you expect to draw off hot water at a rate of 15 litres/min @ 55C, the store will deplete within about 10-15mins (i;e from 80-55)
It could be a number of things, the boiler could not be firing at the appropriate times, the boiler could be undersized, the store could be undersized etc etc
So without the basic design principles you are going to struggle to understand where the fault lies.