I wondered why sometimes the effect was quite obvious whilst other times it frankly disappeared. Is there a certain time of day when you should stop using them i.e. around dusk when although there's a deep blue sky (theoretically of course...this being the uk) this type of polariser would add too much of a tint/darkening to the footage? I was using one the other day (during a rare magic hour) and I felt my footage could've been a little brighter. However, thanks for that tip I'll definitely bear that it mind for the next shoot.
Without going down the mattebox route (reading through some old threads about the subject on these forums suggest that's a bit of a minefield also...especially considering the cost of a decent (eg chosziel) kit. Wondering if I'd get away, in the meantime at least, with adding another screw on (ND this time) to the lense alongside the formatt polariser.
All in all, this past week has been a tremendous learning curve...glad I'd aksed the questions and really appreciative of the responses.

Thanks for the link Adam.