Thanks all for the advice, I did wonder if there might be more to it than I initially thought.
So it's a 2 bed, insulated with DBL glazing. Double panel rads. 80s build. It's small though, tiny.
I suppose I don't like the idea of a tank because-
1) it takes up space, we want to convert the loft in a few years.
2) the idea you can run out of water. All 4 of us shower in the space of 2hrs. In the evening.
3) I don't like the idea of spending energy keeping water hot for 22hrs in a day, completely unused. I only need it for 2 hrs. Out of principal this seems very inefficient.
There may be a very good reason no one does this, but id rather have ashp for heating and on-demand hot water for when we need it. (Ashp+electric shower+ 2x under sink heaters)