Thanks for all you replies, yes the 315 is an old unit, it was a cheap introduction to geocaching and it seems to work well for my needs,
I was using GRB36 datum dont know why, i just associated it with GB or UK, I have now changed it to WGS84 and will see if I have more luck with that,
the problem i have been having is it sends me to the wrong place, if I google the OS grid refrence it gives me it correctly puts me in my back garden, but the log/lat put me a 2 doors away in my street, it may be the age of the unit or more likely something im doing wrong or even googlemaps,
I did make a homebrew seial cable for it and got NMEA out but not tried uploading to the unit yet, maybe thats the next thing
Thanks again