Why is there no hot water from sink tap unless shower or bath has been run?
I think you are right.  There is a minimum flow rate required before the boiler will actually kick on.  I have exactly the same thing with my tankless hot water heater.

Aside from getting a separate "point of use" hot water heater for just the sink, The only way I'm aware of to get around it is to turn the water all the way over to hot and on full blast.  That usually gets enough flow going to start up the water heater.  Once hot water starts coming out, you can adjust the temp a bit (at which point the water heater will probably shut off, and all you have is the hot water left in the pipes).