I like Streetmap which (unlike Google) uses OS mapping, see http://www.streetmap.co.uk
There is an option at the bottom of the webpage to cut and paste the coordinates, eg.
http://streetmap.co.uk/map.srf?X=534345&Y=168965&A=Y&Z=120
and change the map scale to suit.
Eg http://www.streetmap.co.uk/map.srf?X=534345&Y=168965&A=Y&Z=110
Proper OS maps, can't beat 'em!
Cheers,
Keith