Thanks everyone for your advice, and thanks g8ina for the offer of physical help.
I took the main driver out and had a peek inside, and things got complicated. On the back of the plastic plate that holds the terminals was a circuit board which does the crossovers. I could see what I needed to unscrew but it was fiddly, with the crossover board being exactly mid way between both drivers so going in through either hole would be difficult. Laso, the ends of the terminals on the speaker side did not look like anything that you get from RS or Maplin. So I took it to my local electrician, who had some terminals in stock. He said that once he'd got into the speaker, he realised they were the wrong ones, and had to source some more that would do the job.
And so, I have the speaker back, it's all wired up and amazed at the difference between them and the bookshelves I put in as a temporary replacement. All hail big speakers!
I fully expect something else to happen to break them now, rendering all this faff redundant.