Brown seemed to be thirsty (a sooty tailpipe and a full tank not lasting far enough were the warning signs) so after filling up with petrol (pay day today) I got my tools out. I'd already read up the procedure in the BL workshop manual as the Haynes doesn't cover SU-carbed 1850s. The first thing I did was to remove No. 2 spark plug and sure enough it was covered in soot meaning the replacement carbs I'd fitted were running too rich. I balanced the carbs, (the front one was getting less airflow) adjusted the idle and then I turned the mixture nuts up until the revs started to drop and turned them back down till the revs just started to rise again so hopefully, I should get better fuel economy.
I've noticed that petrol prices have started rising again (96.9p a litre at my local Morrisons) so I'm going to do something a bit radical in order to improve my mileage. So far my best average has been 28mpg. I don't want to say too much at this stage as it will be experimental. If it works then I'll reveal all in due course.