Removing glue from perspex?

[Kelpie]

So, being a complete tightwad I was absolutely delighted to relieve a skip of some old sign boards the other day. They're made of clear plastic of some kind, with one face covered in a signwritten plastic film.
Most of the film comes away quite well, but in places the glue seems to stay behind.
I've tried hot soapy water but it doesn't really budge. Afraid to try anything more powerful. I don't know exactly what type of plastic the sheets are made of, but I believe that the commonly available types (polystyrene, acrylic, polycarbonate) all react badly to things like white spirit, petrol, or acetone.

Any other suggestions? Is there anything that can be done differently whilst removing the film?

 

[PaulRainbow]

Automotive panel wipe.

 

[Daverw]

WD40 works very well at removing stuff, much better than the penetrating oil most think it is for

 

[Kelpie]

WD40 works very well at removing stuff, much better than the penetrating oil most think it is for

That worked an absolute treat, thank you!