Thanks everyone for the thoughts and insight into this. I've managed to try a few of the related dual action sanders to the Rotex family, but both the Bosch and Makita versions only come in a 6" pad and they are really too big for my needs, hence my earlier comment about looking more at the Rotex 125. I assume that sanding discs from other manufacturers can be made to work even if the holes don't match up if I want to avoid the higher cost of the Festool pads.
For a purely flat sander, then the RTS 400 looks like a good bet but would struggle to clean up more challenging boards so I think the 125 is where I'm headed. If Bosch or Makita did a smaller version the cost saving would be very tempting, but I don't fancy that big head burning through thin British boards so for a dual action it looks like Festool is it...
I'm really enjoying the posts on here, lots of interesting thoughts and ideas to try out when I get the shop kitted out properly.
Thanks all - have fun and be safe.