thanks for the checking procedure Ian........... ive had the old girl revved up a few times before, either warming the engine or doing other stuff, and the tacho needle sways this way and that -- only slightly, but like you say, maybe 100rpm each way (or total - depends on day) ...... guess the lambda IS fried then, hence the often disappointing consumption!
rats another thing to fix
(more pressing trouble right now though is possibly shagged alternator and/or lead - rev it up to 2500rpm, turn on lights/fan/stereo/etc and watch those revs FALL FALL FALL...... bring it back up to 2500, turn it all off, end up easing off to stop it redlining!)