How can I replace diaphragm without my instruction manual ?

My Vaillant ( think it's a turbo as has black panel with green knobs as described by others ) is ten years old and hot water has recently been playing around ; problem is having turn hot tap on and off to get boiler to fire up for hot water ( heating on or off not an issue - yet ) but just recently we've had to turn boiler off then on on the green dial on black panel to make hot water work, then next time water just fires up like it should.
I'm a builder , so spoke to blokes I know at Plumbcentre and they said try replacing the diaphragm, so I bought one for the princely sum of £5:90; prob is I.ve lost instruction manual so can't see where and how to exchange part.
Help ?
Am I wasting my time ?
Is it likely to be diverter valve.
Any advice appreciated