JohnH wrote:

I've just looked up about electric combi boilers, it seems this is what
I would need, assuming it pumps out hot water at a decent rate.

The trade does appear to use the name "electric combi boiler" for what
might be a suitable heating arrangement in your - unusual - situation. I
came across such a thing in a holiday cottage we rented a few weeks ago
(http://www.lindisfarne.org.uk/retreat/index.htm - highly recommended!
;-) As the URL suggests, it's on Lindisfarne, aka Holy Island - off the
Northumberland coast, access via causeway at low tide only. Thuz, no
mains gas; property intermittently occupied by succession of short-stay
visitors who can't be relied to light boilers, change gas bottles, or
any such; alternatives presumably oil (but no front garden and back
garden small, no access from any street!) and electric.

So they'd fitted what the mfrs literature called an "electric combi",
though more accurately it'd be an "electrically heated heatbank" -
there's a storage cylinder of (at a glance) 200? litre capacity, which
was heated through a pair of electric elements (use one for summer, both
in winter), time-controller with preference for heating during
cheap-rate night-time but would boost with pricey daytime leccy if
needed (OK for tenants paying multiple-hundreds for a week's stay).
They'd a good programmable stat, whose default setting was more or less
as a frost-stat (5 degs) with an early-morning keep-the-chill-off
period; tenant override by pushing up the setpoint (with resets to 5degs
programmed every 12h, to avoid using up the heat when the place was
empty). HW on direct-mains (unvented) heat-exchanger off the heatbank,
wasn't interested enough to work out whether the CH feed came off a
heat-exchanger or circulated the stored water - but in either case the
CH was very responsive.

With only two bedrooms and a bathroom upstairs, and a largish open-plan
living area downstairs, this arrangement was perfectly adequate to keep
the place toasty-warm on its own - though we dig bring up three
office-moving crates full of wood for the multifuel stove for the
prettiness off it, and in case of powercuts. The tenant instructions
asked us not to fiddle with any of the controls, and so I avoided the
temptation to switch to the 'winter' position (both heating elements). I
believ therefore that the heating (and the bath filling) was done with a
single 6kW element; if one was living there permanently, with more of an
eye on the running costs, I think you'd want to make use of the 'winter'
position to get both elements going in winter, purely to get the bulk of
the energy consumption done on cheaprate. The place was reasonably
insulated - gert thick walls, well-fitting windows - and being terraced
had only the front and rear exposed; but exposed to seriously chilling
winds - this being an island in the North Sea, after all!

What kW would be needed for an acceptable shower and for running a bath?
(won't be used for heating as there are electric rads here).
I've seen some boilers on discountedheating.co.uk from 2kW to 12kW.

Hope the thumbnail above suggests hope, in that case - 2kW sounds
stupidly small, 12kW may be overkill (but would give you relatively fast
recovery, at the cost of needing a hefty feed (50A circuit, may need to
get yor supplier to uprate their incoming fuse).

Can anyone recommend any decent plumbers in high wycombe?

Rocking-horse droppings come to mind ;-)

Ob. thread-drift (but still d-i-y): we drove onto Lindisfarne on a
Saturday, parked the car, and didn't try to move it till the Friday.
Started just fine, but wouldn't move - felt as if it'd been clamped;
well, it *would* move, but only by dragging the non-rotating rear
driver's-side wheel over the gravel! Turned out the sal****er spray +
damncold winds (this was February) had made the handbrake shoe seize up
(big clue: handbrake handle inside the car wouldn't come back up once
released). While waiting for the RAC to show up (hey, it's paid for, may
as well use it) tried - on the suggestion from the man in the Local
Shoppe ;-) - tapping it free with a suitable hammer substitute (wheel
wrench, since you asked - I'd jacked up that wheel and taken it off, for
better access and trying to work out Wot Wus Going On. A couple of
non-vicious taps, a satisfying 'ping' of releasing spring, and the wheel
turned freely! Result! RAC callout cancelled, off to Berwick...

Cheers, Stefek