In your circumstances where waiting a week or two for a machine repair might be an annoyance but wouldn't be a significant problem, I'd say it's not worth it even at £99 (perhaps especially at £99) unless:
- it has a useful feature/stitch/facility that your main machine doesn't have and which you've occasionally wished it did,
- it handles heavyweight or lightweight fabrics better than your machine,
- it is lighter and more transportable than your machine.
If it doesn't bring something extra that your machine doesn't have or isn't a significantly higher quality used machine than your current machine then it's just going to sit there and not be used which, if it's a computerised machine, may prove to be problem if you haven't used it for a year or so when you eventually come to need it. In the case of the used high quality machine, however, you may well find that your main machine soon becomes your backup machine.
If you don't already have an overlocker, buying one (as BrendaP suggests) is a really practical idea.