I decided that I'd find or adapt arms to fit the car and got searching. I've measured numerous cars in the meantime (with some strange looks from passers by) but only found an off the shelf arm suitable for the passenger side, a 207 driver's side arm (part no. 6429.AP); it's about 5mm longer but really not worth it to make it shorter. I have a spare, brand new, one of these if someone's interested in doing this, pm me.
For the driver's side I found a 'GM' arm at a breakers and shortened it down by 3.75" and twisted it so that the pin is at the correct angle to the windscreen. This arm has a long first section (the part that's bolted to the car), this is necessary to allow everything to clear the bonnet. I'm not certain what this came off, but it was about 23" long, from mounting bolt to mounting pin, I have a feeling it is a new Vauxhaul Astra's driver's side arm.
I then painted both so that they were fresh and matched.
So I have now finally finished them and I have the proper flat blades without the big lumpy adaptor that the retrofit ones have (only took a few hours after I got around to start on them properly).
I have flat blades, 26" and 20" (apparently the Volvo S40 Mk2 uses the same sizes). I bought them from an Unipart shop, GWB926F and GWB920F.
Both wiper arms are Bosch manufactured, so they complement each other nicely.
I never managed to inspect a Volvo, it could well be that the arms off the S40 Mk2 would be a direct fitment, maybe one of you could have a look and measure them.
Here are some pictures:
Arms and blades fitted:

Nice gap between the two blades:

Long first section of the driver's arm, (very similar to original):

Picture chowing clearance between the 207 arm and the bonnet, I think there's a bit more than what's shown, camera was a bit low:

I hope this helps anyone else who's interested in doing this nice little update to the car.