Glad the thread is appreciated(!) Few things:
1) Re: the saddle. No, the click (I say this as opposed to creak as it is a distinct double sound of 'tick tick' on each revolution) doesn't occur when I'm off the saddle. I guess that means it could be the saddle, doesn't it? I will grease it and see if it makes a difference. Oddly enough, when the bike last came out of the shop - checking out the click unsuccessfully - the saddle was set really high. Insignificant coincidence but I'm 6 3" and this saddle was high!
2) I don't know whether the sound is drowned by traffic most of the time but I do kind of get the impression that it isn't there all the time. By the end of my journey to a from work - say 20 mins each way - it's definitely there.
3) For a minute I thought it might be the cycle computer - and got all excited ... well, sort of. It's wireless and makes a small sounds on each revolution as the magnet passes the sensor. Anyway, it ain't that.
4) Re: insisting at the bike shop. The shop is Evans - anyone London based will know it. I can't fault them for their attention in the past - exemplary - but thanks for the address of C'dale Customer Services. I'll try them.
5) Only just had SPDs fitted so neither them nor the pedals in general seem likely to be the culprit.
Did anyone read the URL I posted on the main pivot? Any experiences with that?
And finally, thanks for all the replies. When I do get to the bottom of this, I will most certainly update the thread. If it's a joint in my knee or something equally embarrassing, I'll personally seek you all out and buy you a beer.
Best,
Nick