The bath heater leaks and really is huge (but still works after 30 yrs
use)- i'd like something smaller but still electric powered - are there
any instant water heaters solid enough to run long enough to fill a
bath or are they all storage?

You have a choice of a 24kW instantaneous electric water heater, which will
require at least a 2 phase supply. You can get versions that will run one
100A phase (must be the only device on that phase), or 33A each off three
phases.

A 24kW heater will run an excellent shower, but will be slow (but possible)
for bath filling, matching the smaller combi boilers. Don't write off the
possibility of installing 3 phase if you don't have it. Ask your electricity
company for a quote.

Alternatively, you need a storage solution. Practically any hot water
cylinder, whether vented, mains pressurised or heat bank will work. However,
they will take up much more space. Specify one with two immersions that can
run simultaneously for faster recovery. You'll need at least 120 litres in
size.

Christian.