You are right that only a 6 figure GR is given by "Getamap". However with a good resolution monitor together with a bit of patience and manual dexterity you can roughly "plot" the outline of the 100m grid square in question on screen using the screen co-ordinates in the box. I use a CAD overlay grid which is entirlely scaleable to any degree and this allows me to zoom in on the overlay to get a better plot. Result approaches a 8 fig ref which can be "stretched" by guesswork makes it more usable. A bit Heath Robinson but it works for me as I don't need absolute cordinates for my purposes.
Being a natural Scot I do not beleive in paying a penny over the minimum to achieve a goal. That is why I have been using the OS freebie which covers the whole of GB at 1 : 25000. From this I can easily establish waypoints using their digital mapping at zero cost. Just a tad irksome editing the established waypoints in the Geko to suit the new desired co-ordinates. However time is on my side so I think it may work for me.