With combis it is all too common that they are undersized, based only on space heating requirement, causing dissatisfaction and a dislike of fitting by plumbers. The HW demand and max desired / realistically expected HW flow requirement assessed, and that also used as a critical factor in boiler choice. (And yes, if there are multiple bathrooms in the house, two people might indeed take showers at the same time - and the dishwasher might be on, too… )
I should have been more specific by specifying the oversized issue is with System boilers. BG wanted to up my 24KW system boiler to a 30KW boiler, even though the existing boiler has no issues keeping the house warm - in fact I was thinking of lowering it.
On reflection, my 1970 boiler was a water only boiler fed by 15mm pipe, so no idea about other boiler types at that time
We twice fitted boilers in the loft in our last property. British Gas hated them and used every excuse possible not to carry out the service contract. Their main objection was the gas pressure that they claimed was too low.
They both worked well in what was a very hard water area. We had a twin salt store water softener (watch for this one @james_n ) that we had to bypass when topping up the boiler. The only problem looking back on it was that the boiler was close to the boarded floor of the (dormer) loft and getting components out was difficult. I believe a minimum bottom clearance is now specified but not all fitters seem to be aware of this.
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Unlike the water, the pressure of which is affected by effectively adding an extra storey to the building, the effect of that height on gas pressure is negligible - but the gas pipe diameter could affect flow rate, especially if it is a longer path to the loft.
Thanks By-Tor. Good tip. Hope the move went well 
Thanks @james_n - all good thank you. Just (im)patiently waiting for dedicated mains to be installed to get my system up and running again.
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