It's all very well being facetious (it does you no credit, but i guess it's fun) but actually a) I'd already read the info on the BA website re dimensions of hand-baggage before posting the op, b) the information on the BA website didn't answer my question and I wondered whether someone might have had experience of similar situation, and c) having since telephoned BA (and not waited that long for the call to be put through) I've now got the answer I needed, plus some additional advice about cameras and batteries that is not mentioned on the BA website - and that I'm not going to pass on because judging from the comments on this thread would not be of interest.

As you say, if specific information about cameras and batteries were needed then one can always contact BA; I recommend doing so, they are very helpful and it's fascinating what one can find out that's not on their website!