I have seen suggestions of using auxiliary electric heaters when it is only a matter of perhaps 10 - 15 days during a full winter season. Might this cover your needs?

Thanks to failures of British Gas to orchestrate prompt repairs when we were still heating with gas, we have two very compact 1 or 2 kW fan heaters plus a convection heater we bought years back. These all stand by in readiness for such occasions - though the Daikin 8 kW keeps us cosy at -7 or -8 degrees as we experienced last Nov/Dec for a few days. Regards, Toodles.

@toodles  Another option just occurred to me: If I get a little notice of the  particularly cold snap then perhaps I could quickly book a beach holiday somewhere hot..

@ianmk13 Perhaps have a word with @davidalgarve 😉 Perhaps he could put you up! Though I should avoid Valencia area at the moment - they seem to have some flow problems.Toodles.

@toodles Are you going into competition with Booking.com? What are your commission rates?

I am rather occupied at the moment, with a big Birthday, a hip replacement operation and a cataract operation due. Otherwise OK!

@davidalgarve I’ve done several ‘big birthdays’ had cataract surgery (and much more too!) but still have my original hips! My commission rates are (behind hand in front of mouth - mumble mumble.)😉 Toodles.

Posted by: @toodles

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I have seen suggestions of using auxiliary electric heaters when it is only a matter of perhaps 10 - 15 days during a full winter season. Might this cover your needs?

There is sometimes quite a good argument for this and many heat pumps do have the capability to drive electric heaters inline.  Check actual specs at design temp & FT, eg Vaillant 7kW is actually 8.6 at -2 and 45, but quite a few heat pumps only give their rated output at a rather restricted range of conditions.

That said the Mitsi 11.2kW R32 unit (and the R290 when they release it doubtless) or the Samsung 12kW unit arent much bigger than eg a Vaillant 7kW so consider these (or other manufacturers ) if they fit in space/noise envelope.  Ideal also do single fan higher capacity pumps, sourced I think from Midea (the R290 ones anyway).  

Remember the BUS requires that the unit can cover the entire heat load, but it seems that 'the unit' includes (at least according to the interpretation of some installers) any linked backup heater.

Finally how are you estimating your boiler efficiency.  It could be more than 100% (up to 110%) if its actually condensing (most don't because FT is set too high) or 75% if its very old.  This makes quite a difference to the conclusion.

2xheat pumps is of course possible, but you wont get it in under permitted development and it will cost a lot in ancillary kit which installers will want to put in, and there will be installers that wont touch it.  Personally I wouldn't go there at 10-12kW, there are some perfectly good reasonably sized (ie single fan) solutions in this range.

Put these into the mix and you should be able to find a comfort spot.  Feel free to ask more particularly as you move towards a conclusion.  Then of course you have to persuade an installer to take notice of real measurements, which many wont (but some will).

As a word of warning - there are those (perhaps the majority if you include installers) who dismiss the 'what does my boiler use' method due to the uncertainties in boiler efficiency and how the boiler is used.   MCS only formally recognises the fabric spreadsheet (although I do somewhere have an email from MCS recognising that it has limitations for retrofits).  So you are a bit out on a limb relying on boiler measurements.  That said its what I am doing for my install (which, following 2 years of hassle with planning, is now hopefully going to happen in the next couple of months).