We're talking about agency work here, Neil. If you had spent any time in the UK biomed scene, you would have been aware that agency techs are often called in to make up the short-falls. Standing in during an absence of a regular guy. Catching up with back-logs of work, clearing the PM, filling in when Trusts cannot attract permanent staff ... there are many reasons. A few months here, a few months there. Sometimes for longer periods (especially when hospitals fail to recruit). And not all are what you might call "attractive posts", believe me!
All in all, Tony is most probably the most experienced tech I know. Anyone care to mention another?
And ... (I speak for myself here) don't forget that there are some people who don't want a so-called steady job! But prefer instead a bit of variety, if only as a stimulus. Life is too short to be stuck in one place for years on end. Horses for courses, as I keep saying.
Don't knock it, Mate. There is a need for short-term contractors within the NHS. Meanwhile, just be thankful that you're one of the lucky ones, still "out there"!