Bit more description for the dimmies pleaseMarty.
Wow a lot of have not tried and telling every man as his dog how it can't be done, whilst here on planet earth the rest of us just do...
Its pretty simple firstly you need to isolate the mains from the consumer unit, ...use a genny switch or and dp isolator. Wire in the genny to one side of the genny switch and the mains to the other then they consumer unit is feed from the genny switch. I have used this and they have an earth connector to provide an earth to the genny, use some 4 core arctic cable to break out the earth to the gen earth point, do not rely on the earth in the plug that's normally just floating around in the gen.- I use the house earth...we have TNS not a neutral earth.
BS7430
7.1.3 Unearthed generators (rating below 10kW) supplying a fixed installation
Where an unearthed generator is to supply a fixed installation it is recommended that ADS is adopted as follows:
a) One pole of a single phase generator should be connected to the installation MET- ie the generator earthing point to the main earth terminal in the house.
b) The MET should be connected to an earth electrode - so an earth rod (TT) at the house or via TNS. If I had a pen earth system whereby the the earth came in via the neutral I would be adding an earth rod.
c) The installation should conform to BS7671 with all exposed conductive parts and all extraneous conductive parts connected to MET
d) The installation should be protected by RCD's
The earth electrode should have a resistance to earth not exceeding 200Ω
The RCD will not provide protection for faults on the generator side of the RCD, and consequently precautions should be taken. - ie the bit from the gen to the main consumer unit with rcd or if you are using a extension lead ( hmmm thats a no from me) its all risks no rcd... ie do not trust a genny that says it has an RCD...it probably does not and if it does will if trip quickly enough.... But a small suitcase one has few exposed metal pieces and some simple precautions - ie don't turning it off or on in bare feet in a thunderstorm...
We have used a dp isolator and and earth rod in the past at a far more rural site with no issues..same earth rod supplied the house and the genny.... In reality most boilers have a mains transformer and that is usually a standard 50/60hz job so it can be supplied all over europe. As for flame detection requiring an earth...give the unit its own earth if its that fussy, if boilers were supplied needing a neutral earth to work half the continent would not have heating.
The quality of the feed we get from the genny is far better than the feed we get from the mains, we use a little clark unit 1100w they put it at 50hz +/-0.1% , my hz meter in the solar inverter puts it rock steady at 50HZ.. the mains is all over the place +-0.14% the solar inverter switches off mains feed in at 0.09% differential. The legal limit is 0.5% variation. A really cheap gen set with no sine wave rectifying would probably have issues...you need a stabilized supply from a genny..we have used a small 500w genny 12v supply + 12v inverter in the past when we we doing a welsh hills build and living in our caravan...even that managed to power a biasi combi on LPG over winter.
Check out the
http://www.gridwatch.templar.co.uk/ a couple of times a day to see the variation in in the hz.
These are pretty good resources...https://www.electricaltechnology.org/2013/11/how-to-connect-portable-generator-to-home-supply.html