In the grid reference, the two letters (e.g. SU) give the 100km square , for most maps this can be ignored without confusion, then the first three figures are read across the bottom of the map from the left side and the next three figures are read up the side of the map from the bottom. Where the two grid lines from these cross is the indicated point. An explanation of this is also given in the key on the map.
Alternatively since you have access to the web, go to www.streetmap.co.uk ,  click to select 'Landranger grid' (note,do not select 'OS grid').  Enter the grid ref given above with no spaces, for example  SU852220,  then click 'Search'.  You should get a map showing the location.  If you are still not sure where this, is click on the -ve sign, bottom right corner, to zoom out until you get a smaller scale map showing you the general area.