OK, didn't work. I let the green bar fill as far as it could (left it for about 15 mins) and it only got halfway across the "X".
I don't know what changed, but now I can't view any shared subfolders on the HTPC. I set the HTPC username folder to share with the homegroup, I can see all the subfolders, but can't open any of them, getting the "You don't have permission..."dialog box again.
Doing it the other way round, sharing from the Laptop to the Homegroup, works like a charm.
Gigabyte EP35 DS3R, Intel Core Duo E8400 3.0 Ghz, Asus EAH5850 1GB, Western Digital 750 GB 7200rpm 16 MB cache, Corsair 4 GB DDR2 800, Thermaltake 650W PSU, Antec 900 Case, Windows 7 64 bit on Corsair 80GB SSD