it's a pain in the neck but do-able...
gat yourself a copy of the OS document on format conversion (cant remember the name, something like 'a guide to co-ordinate systems', downloadable from the os web site and start coding. I did it in vb5 so I don't suppose my code is a lot of use to you, but I created routines to convert WGS84 lat/lon (i.e. what you get from a GPS unit) into eastings/northings and vice-versa. The conversion process is not simple, allow a week of coding to get it right!
bear in mind the limited accuracy of GPS- 5 metres is asking a lot.
You might improve matters by using mappoint calls to determine the side of the road that the address is on?