Combi Boiler Mains Supply Pipe Size...?

You seem to be confusing 'efficiently' with 'effectively' all combi boilers require a minimum cold water flow rate of at least 9/lpm, Easy way to establish this is by placing a 1 ltr jug under your cold water tap, have a stop watch handy then record the time it takes the tap to fill the 1 ltr jug with tap fully open. If for example it takes 5 seconds then divide 60 into 5 this then gives you a flow rate of 12 litres per minute which is more than enough to meet the boiler manufacturers minimum specification, if how ever the flow rate is less then 9/lpm then you will suffer from poor cold water service flow rate. Ask your water supplier if they can improve the situation if this is the case.

Makes no difference, you need to check the mains pressure and flow rate. A lot depends on the size of the new boiler in KW.

Hi Nick 15mm is sound it's just best to have a good flow of water, most houses have a minimum of 8 litres per minute and up to as much as 20 lpm in some areas but it's up to the utilities company as to what you get, low flow is most likely to need a smaller 24Kw combi but also depends on how many rads you have.