I would appreciate some advice regarding my thinking-out-loud
assessment of our heating system, and the best way to improve it
somewhat if at all possible.

I've recently bought a relatively new (2007) 3-bed 2.5 storey semi
which is obviously very well insulated and given the variety of sizes
of radiator appears to have had the heating system designed through
calculation, even if just following rules of thumb. The radiators,
with output figures obtained from similar-looking Flomasta spec
sheets, a

(Room HxW Type Output)
Kitchen 500x700 Double 1048W
Hall 500x600 Double 898W
Downstairs Loo 300x400 Single ~250W? (a real tiny one - I think it
looks great!)
Lounge 600x1000 Single 948W
Conservatory 600x600 Double 1041W
Front Bedroom 500x1000 Single 812W
Rear Bedroom 500x1000 Single 812W
Guest Bathroom 600x500 Double 868W
Top Bedroom 400x700 Double ~1000W?
En Suite 600x500 Single 488W
Dressing Room 600x600 Single 570W

Total: 8735W

The boiler is a condensing Ideal Icos HE15 (I know - it's probably not
the best place to start but it's what we've inherited and it seems to
work well enough so I'm not looking to replace it just yet) which can
modulate between 8.8kW and 14.6kW. It's an S-plan layout run in 10mm
(the branches at least - I note the presence of 22mm at the boiler end
and within the airing cupboard), with a digital CH stat in the lounge
and a sealed indirect cylinder for the HW. The neighbours are only
present at the weekend so for all intents and purposes I think their
house is unheated most of the time.

I have a few issues that I'd like to address:

1. Whilst I have yet to perform a full and proper observation the
boiler appears to cycle a fair bit even before the house is up to
temperature. I am wondering if this is because the total radiator
output (even before any necessary derating) is at/below that which the
boiler can modulate down to? That is, the system cannot get rid of the
heat quick enough so the boiler has no option but to cut out? As
described below I am planning on increasing the size of some of the
radiators - would this help?

2. The full-width conservatory was built after the house, and I am
pretty sure that the 600x600 Double 1041W radiator in there was
originally in the lounge under the window of the then outside wall
i.e. they simply moved it from the inside to the 'outside'. It appears
that this radiator is inadequate for heating the conservatory in
colder weathers and whilst it has not been too much of a problem yet I
think it may well be in the winter. For better or for worse we want to
use the conservatory all year round so should I replace this radiator
with something bigger? Or should I just supplement it with electric
heating, perhaps a portable convector heater, as required? The latter
might be beneficial for those times when we need a little heating in
there when the CH is off (e.g. cooler summer nights).

3. Given the suspected re-siting of one of the lounge radiators into
the conservatory this has left just the 600x1000 Single 948W in the
lounge and it seems to struggle by itself to bring the temperature up
in any reasonable time i.e. the rest of the house seems to reach
target temperature (as set by their TRVs) well before the lounge yet I
can confirm the lounge radiator is very hot to the touch so it does
seems to be trying. I was thinking of replacing it with a 600x1400
Double 2428W (maybe bigger?) thus bringing the heat output in there
back up to 2kW+ as originally designed. Does this sound sensible?
Would it help with the cycling? The stat is in the lounge hence no
TRV. The lounge opens into the conservatory, and for various reasons
we may well want to keep the doors to it continuously open (or at
least partly), and so it is obvious it will also be partially
attempting to heat the conservatory too. I could add an additional
radiator back in there (perhaps where the original second one was) but
that'd be a bit more work - replacing the single radiator would be
much easier.

Any thoughts? Sorry for the waffle, but hopefully it describes the
situation for you to be able offer some advice.

Mathew