The manual and wiring diagram can be found here:
http://www.gasaid.co.uk/?/Manuals/Boilers/Combination/BAXI/4707513/
Take a look at page 24... The two blacks are the switching lines... Ideally, to take full advantage of the appliance, you need a 0-Volt switching clock system (Ie: It has 5 wires including earth)... The clock you have bought has just an output voltage and should be able to be connected directly onto the return switching live black as the boiler uses a 240V switching circuit, but this will disable the summer/winter option on the appliance itself.
GasAid.co.uk, December 2006