Just in case this is of use to anyone else, Maciej (the developer of Permalink Manager Pro) supplied the following code to fix the issue: function w...

Just to let you know, @tutrix , I tried that snippet but it didn't seem to make any difference. Will wait to hear back from the PMP developer.

No but I could give this a go. I've also contacted the developer of PMP to see what he says.

Thanks Tutrix but had already done that, that was how I'd adjusted the slug in the first place. Upon further investigation the issue actually looks ...