I can only endorse your thinking Iain, I think any late tench I have caught in the past have come from commercials. Their feeding pattern is different from wild lakes, maybe something to do with how they are fed, I don't really know but commercials are always a bit different. I would suggest Luncheon meat, it will attract carp but it maybe not be the time of year to be mass particle baiting to get the tench rollicking. A lump of meat for them to come across or sniff out may work better. I would be thinking more of targeting a single fish this time of year and they love luncheon meat. maybe just a few lumps of it around the hook bait but I would be happy just fishing it on its own. If the tench are not feeding then it wont make any difference how much you put in but if one or two make a feeding round-round the lake, they might just be tempted by a single bait than a great mass of it where they might only pick at it and move on and just as likely to miss your bait as they are unlikely to go on a mass feeding spree this time of year whereas a single bait on its own they will be straight on it if they want something to eat but not go on a feeding binge like they might in May or June.
You might get more than one of course.
Which is a bit different advice than Tiggers but its just my own way of thinking with late tench but you could try both with two rods, I imagine your cousin will get some action with something at least. The luncheon meat I would fish close in, about 3-5 ft from the margin. Tench are more likely to be in this band as the waters colder in the middle and 3-5 ft means any swimming inside that might pick up the scent and and any swimming 5 ft further out might pick it up as well so your covering a wider margin, more chance than fishing it very close in. Method, I only fish one which is laying on/lift with a float and as few shot as possible. a quill or something similar either attached bottom only and sink the line if there is surface tow or just normal. You could try floating it mid water now and then just on the chance the tench are up but on the bottom would be my mainstay. But a link lejer and quiver tip if its preferred, I'm just a bit wedded to a float really.