Judith I am sure someone posted on the forum that you could use a site called
www.nearby.org.uk to find postcodes from a grid reference. I have tested this, it gave me a postcode 80 metres from my house, which is in fact the next group of postcodes up from me 80 metres from my house. I can access this website from my Nokia N8 phone, which also has its own basic mapping programme, have used this to find petrol stations and shops when I have been away from home - i.e. it has taken me to Wickes in Bracknell and Sainsburys petrol station in Bridgewater. This bit of the phone has the option to use in walking or driving mode, can save points and I downloaded a trial copy of Viewranger, but have yet to put this to the test!!
As for the TomTom One, I have used one for five years, the first I found in a field, having been left there by a car which crashed through a hedge! When this died after two years I purchased a replacement which was a re-conditioned model from Ebay (half the price of a new one) it came in a box as a new one would, with the warranty and cables etc supplied with a new one.
I stuck with the TomTom because the friend I work for gritting runs two lorries transporting containers all round the country, he recommended the TomTom to me as he has tried several satnavs in the past and found these to be the most reliable, also easy to keep updated - he also uses the traffic holdup warnings and finds these more reliable than warnings given out on the radio which are frequently out of date by the time you hear them!
Linda