Yay! After much fiddling around, I've worked it out. I had seen W2AEW's video, but I couldn't square it with what I was seeing. There were a couple of problems. The first was that the scope seems to turn the intensity way down when you use any of the delayed timebase modes, so that you only see the intensified portion. Once I realised that, I turned the intensity up so I could see the whole wave. The next problem is that, unlike the Tek, the Telequipment doesn't seem to allow you to view both the original and 'zoomed' waveform at the same time. You have to set the portion to be zoomed while in the 'B INT BY A' mode, and *then* hit 'B DEL BY A'. Apart from that it all works like the Tek in delay and timebase terms. There is a 'MIXED' mode which seems to do a half-arsed both-at-the-same-time job, but not as two separate waves.
There's a few things remaining to work out. I still haven't got anything sensible out of the 'B trigger' business. There are also two separate EXT TRIG inputs, one for the A timebase and one for the B timebase. No idea why that would be useful.
On the upside there's both a sweep gate and ramp output (right there on the front panel) and a Z mod input on the rear panel. So that's pretty much full house! I'm very pleased with this acquisition, having suffered Tek-envy brought on by W2AEW's videos.
John