The 'regularity of use' is sometimes questioned. Chlorine or other additives have a finite lifetime in water systems (I seem to remember about two weeks?). Bacteria can grow in 'dead legs' - parts of the plumbing that don't see regular flow. Apart from true dead legs, there are various bits that serve as such in our house. The pipes leading to the bath taps aren't used all that regularly, neither are all the showers (yes, a first world problem) etc. We do try to remember to flush them all through every month at least, but who knows. I assume that if legionella breeds in a water system that you regularly use, then your immune system will have time to develop some resistance at the same time. So I think that is one reason why there is a very low occurrence of legionella cases in people's own homes. Much greater risk elsewhere.

Yes.
The wholesale price of gas has gone up x5 this year (depending whose numbers you believe) and the wholesale price of electricity has gone up x2 on average (excepting the mad spikes last week when there was no wind blowing).

So your heating will likely become more expensive, but not increase by as much as if you were on gas central heating.

Many utility cos buy gas/electricity well in advance, so it will take them time to filter through, those that did not will probably go bust

It has been running for a few days now. Some things I have noted

1. Run time. Although I have put in heat pump settings into Evohome and used the right 'boiler relay', I am concerned it is short cycling. I plan to contact Evohome and Daikin tech support.

2. Daikin App is quite good and easy to use. I am able to turn HW and CH on and off as I wish.

3. Circulating pump makes a noise when ever it runs. Would prefer it to be quieter.

4. In today's light heating demand, when it starts, the circulating pump runs for about 9 minutes before the compressor starts. Then the compressor runs for about 6 minutes.

5. When not calling for heat, the electrical consumption is about 100 watts. I view this as poor for such a modern device. Your TV is required to have less than 0.5 watt in standby.

6. Seems lucky that I am insulated against large gas price increases, but I do expect my Octopus electricity costs to rise too.