I am aware I have to take at least the CPL exams, although for the looks of it I would be more "future proof" going straight for the ATPL exams - am I correct in my thinking that you can take the ATPL exams without taking the CPL exam (unlike in Australia)?
Yes, that's absolutely correct! If you're considering aviation as a career (and not just as a leisure activity) and to be more competitive, then you should definitely go for the ATP! On a time and study basis there's only little difference between the CPL and ATP. So once studying, why aim for the lower-grade license?
What sort of timescale would be reasonable for a motivated chap to work through them all?
Well, difficult to give a definite answer to this question. It all depends on how much time and effort you can or are willing to put in. If you have to work a full time job and have to take care of a family, it'll probably take you longer than someone who has all day for studying. If you want to get it done in minimum time (~ 6 month) and if you can afford it (time & money wise) I'd recommend attending a full time course, like CATS or Bournemouth Flight Training. If we're talking about distance learning, I'd say a realistic time frame would be between 12 - 18 month (if everything goes as planned, but as you know life is full of surprises). You might want to take a look at Bristol Ground School regarding distance learning, the school has a pretty good reputation. But don't underestimate the time and afford you have to put in the ATP studies, that's actually the biggest hurdle of the conversion!