Turns out that it's precisely what's in my laptop that's limiting my speed.
My iPhone sees no decrease when connected via the repeater.
So, pulled the Edimax USB dongle out of my PVR, plugged it into laptop and installed driver. Speed much closer to when connecting through the router.
All well and good, but the dongle seems to almost hang at times.
Furthermore, although my iPhone and laptop's inbuilt wi-fi will switch to the strongest signal automatically as I move around the house, the dongle won't.