As you go up in altitude the boiling point of water will decrease due to the reducing atmospheric pressure. Water boils at 100 C at sea level however at, say, 2000 feet above it will boil at 97.8 F. As boiling water is a good constant it is often used to check the accuracy of the upper range of our thermometers. The other end is calibrated in ice water which will be at 0 C.
To check the temperature that water boils where you live you need to know how far above sea level you are. This is easily done with some satnavs, a mobile phone with GPS or by looking on an OS map. For most of East Sussex will be between 99 and 100 C.
Get your water boiling and then put in the probe. There may be slight deviations depending on the outside weather but it will be accurate enough.
To see at what temperature water will boil at your altitude you can find the details here http://www.engineeringtoolbox.com/boiling-points-water-altitude-d_1344.html