Float on fuel sender keeps coming off! Whats going on?

[Cheeseybeaner]

Hi just replaced sender unit on the Diversion 900 and am having a problem with the float arm and pickup cam basically coming off - losing the little circlip and escaping into the tank. I drained the tank and fished the bits out but its done it again after reassembling it all...
I always thought that little circlip was a precarious way of securing it but its never actually fallen off before.
Am I doing something wrong? I tried to tighten the circlip when I put it all back in there and couldnt see it as being a loose fit. I'm wondering if I've put the float arm back on the wrong way but it goes through the full movement and rests on the stop looks correct to me.
Anyone else had this weirdness!???
Worked fine for about 100 miles then zilch and I can see the bits loose in the tank.