Hot CH but no hot water - sometimes!

[hc]

Help! Sometimes we cannot get hot water, although we still have GCH.
We have a gas boiler in the kitchen, and a hot water cylinder in the loft. We can select HW only, or HW and CH.
The problem seems to be triggered by running a bath.... so is it an air-lock issue?
The boiler fires up, CH is on, the radiators all get hot as expected, but then we have no HW. (Water does come out of the hot tap, but it is cold.)
If we select HW only, the boiler goes off (my partner thinks it fires up briefly before going off, but I'm not certain). We can only get it back on by selecting the CH and HW option. But still no hot water. It usually sorts itself out sometime the following day, either during the timed period the following morning, or the timed period the following evening. I have heard very loud gurgles coming from the loft and then the water does get hot.
Where is the (proposed) air-lock? What can we do to remove it, and how do we prevent it happening again?
Or is it a sticky valve somewhere? If so where? Or an intermittently faulty switch?
We haven't called a heating engineer, because we know by the time he comes, there will be nothing to see!
Any help gratefully received,
H.