First of all, please accept my apologies for my extreme tardiness over providing a conclusion (of sorts) to my problem. LONG POST ALERT:
The ombudsman finally reached a decision, which was passed to E.ON for comment/action. Unfortunately, as both my meters are working, the ombudsman cannot compel E.ON to replace my SMETS1 electricity meter with the SMETS2 unit I want (but that isn't the end of the story; read on).
The ombudsman DID agree that E.ON's customer service had been deplorable, so told them to send me a letter of apology and to credit my account with a hundred quid as a goodwill gesture.
There's more...
I was under the impression that the government wanted everybody to be using smart meters, full stop, from which I inferred that, as I already have them, I would simply have to put up with my current lash-up of a configuration. However, the ombudsman has stated that the actual aim is for everyone to have FULLY COMPLIANT smart meters, which apparently means 'latest generation', i.e. SMETS2, so I should get the upgrade eventually, when it becomes a higher business priority for the energy suppliers to do more installations. The target date is some time in 2025 (!)... and that's likely to slip!
Taking that into account, the ombudsman also told E.ON to prioritise me on their register for (eventual) SMETS2 rollout – and has suggested in conversation with me that, meanwhile, I might try to find a supplier willing to give me the electricity meter upgrade in return for switching to them (which I had already thought of).
As mentioned elsewhere, even when you HAVE a coherent smart meter configuration capable of providing a dual-fuel display on a single IHD, you won't necessarily be able to trust the energy cost it shows you because it is based on tariff rates stored within your actual meter(s), which may NOT be the same figures as your supplier is using! Your supplier CAN update these remotely with just a few keystrokes (it doesn't happen automatically if and when your tariff changes), but many suppliers simply can't be a***d to do it! In my case, E.ON eventually carried out the update... but only after I'd requested it more than once. Again, the ombudsman couldn't compel E.ON to update these figures in good time, but would expect their systems to be fit for purpose and accurate for their customers.
As for IHD alternatives, I eventually settled on the Bright app, which can display in real-time, but only when used in conjunction with their proprietary display and CAD unit. However, there would still be the potential tariff inaccuracy – and I'd need TWO of them for now!
CODA...
Perhaps my main reason for wanting a coherent smart meter installation was that my energy bill had been slowly increasing and I couldn't understand why, particularly as I also look after my elderly father's finances, his energy use is on a par with mine, he is also with E.ON... and both his bill and monthly direct debit had been going DOWN. However...
E.ON had meanwhile emailed me to say that they would be lowering my energy price, but my direct debit would stay the same to help reduce what I owed them. Then, according to my next bill, I now owed them over a grand, chiefly made up of gas use... in the summer, with the heating off!
I suspected that something was wrong (nothing gets past me), and after confirming via the Bright app that I'd used less than a hundred quid's worth of gas over the previous THREE MONTHS, I rang E.ON to complain... again!
Turns out that, since installing my smart gas meter, they had been using the wrong 'units to Kwh' conversion factor. It was about 2.5x what it should have been to begin with, but was now (presumably due to adjustments associated with their recent reduction in price) out by a factor of 100!
As a result, THEY paid ME over £1100. I could also have had my direct debit reduced by about sixty quid a month, but as it's customary to run up a credit during the summer to offset the cost of increased winter consumption, I left it as is.
Suffice to say that I can't recommend E.ON to anyone!
Apologies once again for the delayed response and the long post, but I hope it may prove helpful and informative for some. Grateful thanks to everyone who responded.