So based on the input and recommendations from this thread, it does seem there is prevalent use and significant benefit to be had to adding a sub to our 10" tops in terms of additional low end and allowing the tops to focus on mids / highs without having a big chunk of their work diverted to the energy draining low end. I have narrowed my choices down to two of the lighter 12" speakers:
RCF 702 AS II - 39lbs, built in mixer & crossover, f resp 40hz to 120hz - £696 new
Alto TS312S - 55lbs and almost 1/4 bigger in size than the RCF, crossover, f resp 44hz to 95hz - £344
The RCF is almost double the price of the Alto, but the weight & size differential I suspect is going to make quite a lot of difference in terms of portability of the RCF to me which means it will get used more regularly and, based on the awesome reputation of the RCF alone (as I've not heard either in practice), I would expect there to be a bit of step up in speaker and component quality in the RCF vs the Alto with a corresponding greater clarity / lack of low end 'muffle' for the RCF.
If I could get a vgc RCF 702 AS II used for around the £500 mark that would be a clincher. Any recommendations for good used sites for PA gear where I might be able to pick one of these up?