Here are a few useful websites:

The OS website has got an improved search facility and will take you straight to lots of minor place names. The maps go up to 1:25,000 scale, which sometimes have barrows marked that aren't on the 1:50,000.
http://leisure.ordnancesurvey.co.uk/leisure/ibeCZzpHome.jsp?minisite=10020&respid=22372&grp=IBE_M_OTHER

The Magic interactive map will often give you the scheduled monument details. The sites show up in orange if you customise the display. There's an easier way too if you set your profile/preferences on TMA to bring up Magic links from every TMA site. If there's a place on TMA near to the place you're interested in, you can open Magic through that and then pan around the map.
Magic only gives details for English sites.
http://www.magic.gov.uk/

Welsh sites have a similar map/database here
http://www.coflein.gov.uk/

Scottish sites are via this database:
http://www.rcahms.gov.uk/pls/portal/PORTAL.wwa_app_module.show?p_sessionid=385698&p_header=true

There are others (such as local Historic Environment Record databases) but I think these are the main basic ones, and they have free public access.

I hope this helps (perhaps it is not new information to you). Thank you for your suggestions about Y Pigwyn: the Eds do have limited time to devote to the website and it is difficult to be all seeing. This is one reason why the most recently added sites (such as Big Low) often tend to get attention, whereas 'quiet' sites like Y Pigwyn may get overlooked.

Thanks. Keep the contributions coming.
TMA Ed.