Some advice on altering heating system

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Hi Guys Some time back we added a toilet and kitchen to one end of the house to make a self catering one bed holiday let, the existing system is an oil fired combi for the lot basically 3 beds 2 lounges, 2 kitchens 2 shower/ bathrooms the boiler is a turkinton 90/110, I intend to split the system to provide independant heating, my plan is to use a Amptec or similar for the letting part, 1 bed, lounge,kitchen and toilet shower room, one rad in each, hopefully if this works out do the same with our part of the house, so how are these little boilers in the real world I have heard that they can be costly to run but with the price of oil going through the roof I thought this might be the way to go, any input would be great
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Geordie spark will know