Exin was a Spanish company that produced, among other things, this very good castle-themed construction toy. In concept, it was similar to Lego, but actually far more impressive to look at, in my opinion (at least for making castles!). It seems to have been very popular and rightly so. This video (actually a series of stills) shows the toy well, so long as you turn the sound off and try to ignore the irritating titles!
As to why I'm mentioning Exin here: they also made a series of small figures to go with their Castillos range, including two 'Alf'-type ghosts and a witch on a broomstick. Although no longer in production officially, the figures do seem to be available from Mexico in plain unpainted plastic as shown below:
The ghosts and witch figures would be perfect for a 3D version of the Spooks and could easily be coated with luminous paint. The knight figure on the right would make an acceptable 'Algernon' and there is also a medieval princess figure (not shown), more 'Polly Plantaganet' than 'Tilly Tudor'.
The video below shows a castle with ghost figure. Note the 'burning' torches on the walls - very atmospheric.
Thanks to Kim for bringing Exin Castillos to my attention.