I was involved in the acquisition of a company a few years ago and it turned out a couple of their software developers had not assigned IPR to their employer (this was US). We wouldn't complete without that being fixed. In the end they walked with a couple of million each for fixing that commercial oversight. I remember the CEO going mental at the thought of losing the deal. But without clear ownership of their IPR we were paying for nothing more than some paper and logos.
I think as has been said its reasonable to restrict this to in the course of your employment. If that is not sufficient for them consider a non-exclusive, royalty free, assignable perpetual license to use.
You might also want to look at what this is worth to you. The fact they are in conversation with you and looking for retrospective assignment makes me wonder if this isn't hampering a deal that they have on the table.
You should also be aware that you cannot give them that which you may have already given away.
