R15.13.2 in full (makes the above post more helpful):
Where combination tap assemblies, either with or without a hose and shower head, or shower mixer taps are installed, and both cold and hot water (that is, fluid category 1 and 2) are mixed in the body of the tap, it may be considered reasonable, where pressures are unbalanced, for there to be single check valves on both the cold and hot inlets to the combination tap or mixer as shown in Diagram R15.13.2a [the diagram shows a tap with in-body mixing and single check valve on hot and cold pipework]. Under the previous Byelaws single check valves were required to prevent the possibility, under fault conditions, of the 'cold side' pressurising the 'hot side' and, under vacuum conditions, of hot water returning to the mains 'cold side'. The Water Supply Industry supports the provision of single check valves where there are unbalanced pressures.
Where a combination tap or mixer with water mixing in the body is used with balanced pressures (that is, both inlets are fed from supply pipes or both are from storage), the 'cold side' is now rated as Fluid Catergory 1, and the 'hot side' is now rated Fluid Category 2, and single check valves should therefore - according to the Regulations - be installed on both 'sides' (see Diagram R15.13.2a [the same diagram previously referred to]). Alternatively, a combination tap with separate waterways as illustrated in Diagram R15.13.2b may be used [shows a mixer tap that does NOT allow in-body mixing installed without check valves].
The Water Supply Industry deems that the particular requirement of the regulation will be met if there no check valves on either 'side' of the supplies to 'mixer' taps where water mixes within the body and the supplies are on balanced pressures.
Emphasis (bold letters) is mine.
My reading of this, and that of my teachers when I was at college, is that with balanced pressures, a strict reading of the regs says you do need check valves except that the point of having checkvalves is to prevent the cold side pressurising the hot and the hot being sucked down the cold if the mains goes down, so an interpretative reading, supported by the water industry (so you can't get in trouble for making this interpretation since it's in the "WRAS Water Regulations Guide") is that you don't need them actually, provided that neither side has a check valve.
I personally interpret this as: if your combi boiler has a check valve on the inlet, then there is a check valve on the hot side of the tap, and, if so, you don't then meet the get-out-of-having-to-fit-checkvalves criteria.
