Well, I usually use the magic map
http://magic.defra.gov.uk/
- you have to 'choose your own topic' first and select 'scheduled monuments'.

Also depending on which county you're interested in, there are various historic environment record interactive maps, sometimes they have more detail on. The Heritage Gateway has got a handy list so you can see which ones have their own sites
http://www.heritagegateway.org.uk/gateway/chr/default.aspx

- like Somerset's got its own and I rather like it
http://webapp1.somerset.gov.uk/her/sop.asp?flash=true

but there's also a kind of catch-all map on the Heritage Gateway, you have to narrow things down a bit to get it to work properly I think?. And you can put in OS references, although it doesn't look like it, you have to click at the bottom.
http://www.heritagegateway.org.uk/gateway/default.aspx

Ok so I may be slightly map-obsessed, Carl.