A good result from the weekend's work, but I can't say for sure what my problem was....
I disassembled each of the calipers in turn and cleaned up any areas of the caliper, mount & pins where there is metal to metal contact to make sure that nothing was sticking.
I pushed the front pistons back into their calipers (with the bleed nipples open) to remove any old fluid from behind the piston that wasn't replaced when I replaced the brake fluid last autumn.
I also rubbed the existing pads over some sandpaper to remove any surface contaminants or glazing.
Put it all back together, bled everything properly and there's an amazing difference.
Much more initial bite. Also, when you're half way through braking to a stop from speed and press the pedal harder, the increased pressure actually makes a difference, which it didn't before.
So, I'm not sure which of the three things made the difference (sanding the pads, freeing up the calipers or clearing the old fluid out from behind the piston), but one or more of these has fixed things!
I'm sure that new performance pads would make even more of an improvement, but at least it feels as good as my old 1.8TS did now and I have more confidence in them than I did before.