Hi,
If it's just to stop the slug of very hot water, another way would be to split the HW from the combi into two branches, a short one and a long one.
As the slug of hot water comes out of the boiler, half goes through each branch. The half going through the shorter branch would be mixed with cooler water coming the longer branch. Then as this half comes to an end, the half coming through the longer branch would mix with cooler water coming through the shorter branch.
Needs no valves, pumps and thermostats, and can be easily retrofitted into existing pipework.
cheers, Pete.