Beech - good to see you've crunched the numbers to make an informed decision, that landed around where I thought it would in terms of the relative benefits. Regarding some of the alternatives also listed, my own brief thoughts are:
-Air source heat pumps do give you some leverage in terms of making each unit of electricity do more work for you, but the coefficient of performance (or COP) is strongly temperature dependent. In summertime (where you can run some of these units as air conditioners) and at the beginning/end of the heating season (where your heat demands are low) the COP is generally quite good - sometimes as good as ~4 (i.e. 4kWh of heating for every 1kWh of electricity). In the coldest mid-winter months however (when you use the most energy), the COP drops off dramatically, sometimes being little better than just running an electric heater. Some units also suffer from icing on the external heat exchangers in really cold conditions; this is dealt with by temporarily heating the coils (running the pump in reverse) which is a further source of energy loss. I've never crunched the numbers myself, but my gut says whether ASHP's are a good idea depends on where you are in the country and what your winters are like. A mild south-west England winter they might be a good idea, if you're in the Scottish Highlands I would think less so.
-Ground source heat pumps give better performance because they absorb their heat from underground pipes (they are only a few feet down - it's not the same as traditional geothermal energy). Because the temperature just a few feet underground is much more stable year-round, the COP of these units isn't so dependent on atmospheric conditions and you get much more efficient mid-winter heating (still with the COP approaching 4 if you don't need high temperature heat e.g. ~35-40°C for under-floor heating rather than ~80°C for a conventional radiator system). You do however need a big area of ground to lay the pipes into (otherwise you just freeze the ground from trying to take out too much heat from a small area) and accept the disruption those groundworks bring. The scale of the installation also makes GSHP's very expensive to install.
-Solar thermal - is great for supplementing hot water demand, particularly in summer. As a method of space heating though it's terrible - solar gain in winter time is very low (which is why it's cold in the first place!) and the size of collector you would need to satisfy a significant heating demand is unreasonably large. Similarly for solar PV.
-Waste oil burners have been around for a long time in workshops; good if you have plentiful supplies of waste oil.
Brtone - you are suggesting that diesel heaters aren't up to much - what do you base that on? My own experience is that this type of parking heater is perfectly capable of keeping my garage warm even on a cold day in January. In my view, whether they are a good buy just hinges on whether it's the most cost-effective heat source (which is a case by case thing).
