My thanks to you all for your words of advice & encouragement, which I'll heed in the future. Bernina or Janome, here I come! 

If you've still got your 720, I've also got one which needs a new motor. If you want to sell it I would be interested in taking it for the spares.

I'm a first timer on this forum & a total novice to overlocking. Several years ago Aldi had Delta overlockers on sale offer & after having a small success  with normal machining, I thought I'd give the overlocker a try. For various reasons, (not least was it frightened the life out of me), it stayed in the box & kept the rest of the junk in my loft company. A couple of weeks ago I thought the Covid lockdown would give me a good reason to get it out, so out it came. The last fortnight, the machine has caused me nothing but grief & frustration. Admittedly, some of it is certainly down to my almost total ignorance in the use of overlockers. On two occasions, the front feed dog attachment screw has has become loose, causing the machine to stop feeding completely! I've totally re-threaded probably fifty time & also replaced the needles. The thread tensioner wheels were all askew & one actually decided to disassemble itself whilst I was turning it. The latest problem is the L/H needle thread keeps breaking. As an overlocker, I think the machines a 'wrongun' although it's really effective for hair removal (by the handful)! As you've probably gathered by now, this post is a cry for help! In better times, I would have seeked out somebody who knows a lot more than me locally (which is almost everybody) & asked their advice, but the pandemic prevents that. Help & advice from any quarter would be soooo much appreciated.
Sorry for the rambling, but I was completely sane a fortnight ago - Honestly!