OK, well, to update. The overlocker is now sewing although the tension is far from right. I cannot though make the knives cut - the top one is pushing the machine under the foot. I've scoured the internet looking for info, but there is little specific to the knife settings.
There's and outside chance that there may be a spring missing from the top knife, but I'm not sure. I found a spring on the table, and I'm now wondering if it's come from the top knife assembly - it looks about the right size. I'll have a look tomorrow....
If we have any overlocker geniuses (genii?), who have had this problem, and solved it - please point me in the right direction!
The one for sale down south turned out to be a no-no as it was only a three thread.
Jessie