Geoff, you can adhere your baseboard directly to the concrete with Liquid Nails.
Put a couple fairly heavy beads (1/4" will do if the wall is pretty flat) of liquid nails down the length of your baseboard, keeping away from the top edge about 3/4", or just below the offset line, if there is one on the back of your base. Press it to the wall to transfer the glue, then tilt the base back from the wall a little. Wait a minute or so, then press the baseboard back against the wall. Tap the lenght lightly with a rubber mallet, or massage it down by rubbing with a soft rag.
You may need to block the base into place in a few spots if the wall is uneven, but within 24hrs. it will be quite solidly attached.