I have had a Worcester Bosch Greenstaer 30HE Plus condensing boiler for over 2 years and it is perfect and fault free. Condensing technology is new to this country but mature on the continent where it has been used for 15 years. I know that conventional boiler prices have collapsed and lots of people are trying to get rid of their stocks as they become largely un-installable after April next year. I wonder if this is driving the second engineer.
Installing Central heating into a house is a big job. To replace my system with all the box work and pipe runs in place took a week. If none of this where there and there are concrete floors in the property then it will be a big disruption to the home!
On the Heat pump side I cant imagine it would be impossible for system to work regardless of the heights of the garden vs the a house. These things work with 4 storey houses so the pumps must be meaty enough to pump to massive head heights. But if the garden is too small have you considered a vertical rather that horizontal arrangement of the collector pipes?
The other option is pellet stoves. These can work with hot air ducting and can also supply hot water. The are self lighting and thermostat controlled and really efficient 90-95% and their internal hoppers allow fuel for continual use over 70 hours for 30-40kg pellets apparently 2 carrier bags full. One could see that with cleaver placing two off these could heat a property easily. Supplying ducted hot air through small holes in the walls to a number of rooms. You could also combine a pellet stove with that heats water with Solar water heating system on the roof,
You still have the pellet supply issue but this seems to be a temporary thing as many new suppliers are opening up all the time. You would also have to lug some pellets into the house ever four days or so, if they are working all the time.
I hope these musings are useful.