I took the plunge and finally managed to get the feed dogs raised on my Janome. I think it has done the trick as it seamed a lovely hem on my top.
I was expecting it to be tricky as fabric (viscose jersey) caused me grief on my overlocker (unusual) and I was starting to run short of a matching thread.
I therefore decided to leave the previous threads on the machine to try scraps and alter tensions, then hopefully I'd not need to do too many tests with the correct thread. It worked so I'll remember that, as at times I've used a lot of thread just trying to get tensions right etc.