Yes, I did this a few weeks ago. The combi instantaneous hot water
supply was used only for a washing machine & a 300 litre unvented HWS
cylinder was installed. If a pressurized unvented DHW storage system
is to be fitted, then you need to control it's temperature with a
2-port spring-return valve. A 3-port mid position valve on it's own is
not suitable. See the Honeywell S-Plan (two 2-port valves) wiring
diagrams at;

http://content.honeywell.com/uk/home...20S%20Plan.pdf

You will get a suitable 2-port valve with the unvented cylinder
package. The installer should have passed a suitable training course
(CITB, IoP, etc) traing course on unvented HWS systems.