Belatedly, no, but I think our installer may have (possibly as a result of the free money!) wildly overspecified the tank which I suspect we will never empty even if we actually tried.

Daikin (and presumably others, but they're the ones we went with) do have integrated tanks/heatpump internal units. It might be worth having a think about your actual needs and it may be that there is a solution.

My experience with electric hot water taps is that they've been universally awful and only a last resort, but equally the state of the art may have moved on since I last looked!