Dig the bulbs up and put them in your brown bin

Thank you for your reply. Although to be honest we tried that but must have missed a couple and before we knew it they had spread again. We will just have to be more thorough this time.

I have had that problem in the past, pretty though they are. It's exceedingly difficult to get rid of them because you only have to miss the odd one or two bulbs, as you've found, and before you know it you're inundated with them again.

If there isn't too much in the bed they're in then the best thing is to dig over the whole area and hope you've been thorough. If you can leave it empty for a season then you can see when the next ones emerge. This isn't always possible though.

Sorry I don't know of an easy solution to this problem.

I assume that the Bluebell in question is that pesky hispanica variety..? - If it is the the native british bluebell (Hyacinthoides non-scripta), then leave it alone. :)
The invasie spanish bluebell is a case of good intentions gone wrong. Lovely in its own right, but flawed in its nature (for British soil).
Keep digging them out. I have the same problem; Five years on and I am still at it.