If your bottom is properly inserted and the zipper appears to be closing, but then opens again behind the pull tab,...I would suggest having a look at the pull tab to see if it seems to be spread open. I have many times pinched the tab a wee bit at the smaller end to make it close the zipper teeth together. Eventually tho, you might need to replace the zipper entirely. However, if you are careful with your pliers, pinching just a little at a time, and take care when unzipping, it should have quite a good life. Hope this works for you.

THANK YOU THANK YOU dancingirl!

On your advice, I just tried pinching the back end of the pull tab a bit with small pliers and it worked brilliantly! It's a little tight to get the zip started now - but the two sides hook up again which is fantastic. (A little wax, graphite or non-oil lubricant can likely smooth out getting it started.)

I was nearly going to give up - thanks again for the assistance :-)

!! So I can put my tights and cape away until another day! :-)