Page 28 of the Instruction manual -Installation Commissioning and Servicing.
I am familiar with this boiler as I have fitted several - including on in my own home.
I am NOT familiar with the
Drayton LPIII time switch.
I am at a loss to understand why the customer does not want a thermostat, but ours is not to reason why.
I am not sure why you propose using 4 core flex. Have another think about it.
Ok that's that out of the way so let us begin.
Is the spur already in place? If not here's an elegant way to do this if the time switch will fit onto a back box / pattress box.
Get one of these: http://www.ebay.co.uk/itm/35mm-DOUBLE-DUAL-PLASTIC-SURFACE-MOUNT-BACK-BOX-TWIN-2-GANG-WALL-PATTRESS-UK-/310378611131
and fit your FCU in one side and your timer in the other. (use recessed metal version of this box if you wish.)
Get a lump of FIVE core flex and use 3 of these for your L N E from the load side of the FCU to the boiler L N E terminals. This is the power supply for the boiler, and at this point with the FCU in the on position the boiler should fire up. The flex should exit the FCU through the normal flex outlet of the FCU
Use some offcuts of the 5 core to connect the L N E on the timer to the load side of the FCU - this powers the timer to make it go tick tock.
Use 2 cores of your 5 core and connect one to terminal LR and the other to LS at position S10 in the boiler (see page 29 of the manual) There should be a link factory fitted here - remove it and stick your wires in instead.
Now go back to the timer. there should be 2 terminals there which you'll connect the other ends of your wires connect to LR and LS in the boiler. Remember, I am not familiar with this actual timer, but it can't be much different to any other.
So now when the FCU is switched on you will have power going to the boiler and it will fire whenever a hot tap is turned on.
You will also have power going to the timer so now it should be going tick tock and when the timer calls for heat the boiler will fire and the radiators should get hot.
I would have posted pictures & diagrams for you but I'm crap at things like that, but hopefully you'll understand what I have written here.
If you don't have the manual you can download it as a PDF (I think) from the Worcester website.
Be careful how you route the cable in the boiler / pipe area - the hottest one is the 22mm on the left as viewed from the front.