However, I am not convinced because it still draws 6kW which my 4kW solar cannot provide so I am paying at peak rates for part of the consumption. Also, it is only just giving reasonable water temperature for 7:00am showers

Not really.

E-REDES did a very rudimentary meter check which proved nothing.

With help from MelPump I have obtained heat pump consumption from MelCloud  over the exact period in question, (2,020kWh) and similarly extracted a figure from my heat pump Shelly EM3 (1,900kWh). It seems that I can't expect any better accuracy from either.

However, whichever I use of these takes me nearer to the billed figure of 2,554kWh and MelPump suggest that the greater consumption could be down to: a) colder and longer winter and b) the poor pipework mods to change the buffer tank to a volumiser, which resulted in a reduced available flow rate. (Almost wish I had never started on the buffer tank exercise!)

So I have more or less given up on this battle and dreading the new 2 month bill expected any time soon.

We are now on MelPump Summer settings and more solar being produced, so I am starting to plan doing the pipework improvements. I will have to use Plex again and I intensely dislike their sharp elbows, but there is not much copper availability here now. I am rooting through my parts boxes to find any larger radius screwed brass fittings.

Unfortunately, I have to have a hip replacement operation so don't know how all that will fit in!

My contract is with EDP, but when Solar PV came along they split off the entire distribution business to a new company, E-REDES and all questions of import/ export have to go through them even though there is no contractual connection.

I have a fight with them currently after a very unlikely bill in February and with a meter check tomorrow.

Same here, you buy (or sell) electricity from/to one company (an electricity retailer), but its another company (the distribution network operator) that has to agree to solar/battery connection.  However the regs oblige the DNO to agree to connections up to a certain amperage and oblige the larger retailers to pay for exported electricity (albeit at a lot less than 12p/kWh). 

This seems to be a case where the UK got it right.  Better not let certain sectors of the UK media know, it will spoil their fun.

There is another factor which I think smells somewhat.

My contract is with EDP, but when Solar PV came along they split off the entire distribution business to a new company, E-REDES and all questions of import/ export have to go through them even though there is no contractual connection.

I have a fight with them currently after a very unlikely bill in February and with a meter check tomorrow.

1. Our electricity suppliers have standard outgoing tariffs, which we pick from.  We can’t negotiate them, only choose one.
2. We certainly don’t invoice the electricity company.  Every month, they send us a bill, which may be positive or negative depending on the time of year.
3. We can be taxed as individuals, rather than needing to set up a business, which is taxed.

Given what you describe, I can’t imagine that anybody in Portugal exports their solar power to the grid, which sounds odd, especially in such a sunny country.

Regrettably it is not a practical proposition.

I was given 5 steps, of which I might have attempted 4, but the 5th one reads:

5 Opening a business with the Tax and Customs Authority which you can do using number 35113 of the Economic Activities Code (CAE), or number 1519 of the IRS code (CIRS) so that you can issue invoices to the company that will buy your surplus energy ( we recommend seeking an accountant to assist you with this part). The amount invoiced will always depend on the contract you negotiate.

I am definitely not getting into that burocratic quagmire!

DHW eco mode (does it exist?)

It does exist yes, again a slightly different response depending on generation of FTC but a small frequency limitation and longer heating time 

but it can be turned on/off/timed from a pretty easy to find point in the installer menu.

This on/off/timed is the same as the Ecodan - but it knocks a few Hz off the highest frequency but the major action seems to be to reduce fan speeds which has more impact on the noise.

The principal effect will probably be ramp ups, which will be slower as a result

Absolutely correct, it helps this hard coded logic of ramp up to a high frequency for ~10-15min (which introduces a risk of overshooting on some systems)
In DHW it lets me run much longer (when there is no conflicting demand for space heating), which appears to be 50-70% more efficient and keeps a low input power which is excellent for the solar

If there is a low noise/quiet mode available from the FTC

There is on the FTC6+, under Service > Heat pump settings > Quiet Mode (1 > 3 bars) 
It should be noted that this setting does require a supporting outdoor unit and while the highest settings reduces a few Hz its mostly about fan speed

DHW eco mode (does it exist?)

On my Vaillant NR/Quiet mode mostly limits compressor modulation.  Deep in the installer menu the limit is adjustable between max 40% and max 70% (which is annoying to me because max 80% would be perfect!), but it can be turned on/off/timed from a pretty easy to find point in the installer menu.

The principal effect will probably be ramp ups, which will be slower as a result

Absolutely correct, it helps this hard coded logic of ramp up to a high frequency for ~10-15min (which introduces a risk of overshooting on some systems)
In DHW it lets me run much longer (when there is no conflicting demand for space heating), which appears to be 50-70% more efficient and keeps a low input power which is excellent for the solar

If there is a low noise/quiet mode available from the FTC

There is on the FTC6+, under Service > Heat pump settings > Quiet Mode (1 > 3 bars) 
It should be noted that this setting does require a supporting outdoor unit and while the highest settings reduces a few Hz its mostly about fan speed