After the last post on this subject, I've made two more phenolic beers in a row.
I've also done some more research, and received some more advice.
One of the beers was taken along to the London Amateur Brewers, who told me I had 4 ethyl phenol, which is definitely a wild yeast or bacterial infection rather than a burning of grain, oversparge or other methodical error.
So my next steps so far have been:
Replace all syphon tubes and taps
Replace fermenters, airlocks and bungs.
Buy new mash tun (never used one before) and throw away grain bags.
Take boiler to pieces as far as possible and soak all of it in steriliser then sodium metabisulphate.
Take hop strainer out of boiler, examine it, take it apart, swear at it, throw it in the skip.
So for my next two brews, the only equipment used in common with any previous brews is the thoroughly sanitised boiler. I *think* the beers smell ok so far... Assuming they don't turn foul in the next few days, I'll be bottling them on Tuesday. Fingers crossed again...