Thanks all, some very useful info there. I had always assumed that thicker oil reduced oil consumption but I've a feeling I've read somewhere (and I'm damned if I can remember where!) that it actually can have the OPPOSITE effect! Something do do with clinging better to the bores and not letting the scraper rings do their job. Did I imagine that?!
Bit more about the car. It's a very early (02) JTS and was bought as a cheap runabout for Mrs A - who isn't in the least bit perfomance-orientated. It was cheap because it had thrown a rod and the PO fitted another engine from (I'm told) an 06 car that had been crashed. It definitely had supermarket 10W40 in it because he gave me the rest of the can in the boot for topups. Having just got rid of Mrs A's 1.8TS, which always used 10W40 (and plenty of it!) I didn't think anything of it at the time (other than the fact that it was cheap no-name oil). It was only recently when I decided to do a change that I looked in the handbook and noticed the rather small footnote "for decidedly sportive use of the car, fully synthetic Selenia Racing 10W60 engine oil is recommended. For JTS versions use only SAE 10W60 synthetic oil. Recommended product Selenia Racing." So, as the engine hadn't burned a drop on 10W40, I saw this "bargain" 15W50 Mobil 1 and thought I'd give it a go. The rest, as they say, is history!
I'm also very interested in Rizla's comment that the oil consumption when DOWN after changing the thermostat. I'd have thought the oil would be even thicker if the engine was running too cold?