Well I've installed the oil tank, boiler and large central heating system. Many thanks to all who gave really useful advice on aspects of it.
So last question :
It appears to run well. It's a Grant BoilerHouse 90 non-condensing as the existing septic tank is near a stream feeding a reservoir for a local village so any failure of the septic tank action due to the condensate could quite literally have been fatal so I'll live with the higher fuel costs.
Should I commision it myself by hiring the appropriate gear or get an OFTEC registered person to do so ? If I do it myself, how awkward or probing is the BCO likely to be ? If an OFTEC person, how long should it take/much should it cost ?
Many thanks