Is it this one ?
http://www.argos.co.uk/static/Product/partNumb er/3321195/Trail/C$cip%3D1500006228.Sports+and +fitness%3EC$cip%3D1500006253.Bikes+and+cyclin g%3EC$cip%3D1500006267.Leisure+cycles.htm
26 inch wheels. To be really sure, look on the side of the existing tyres, it will be printed there, e.g. 26 X 1.95
There are 2 common sizes for a 26in tube, you'll probably want the fatter size (1.5/2.2), and there are 2 common valve types, Presta and Schrader. I'd say go for Presta, they take a higher pressure.
The inflation pressure is also written on the side of the tyre, e.g. 35-80 psi. I'd recommend you inflate to the highest recommended pressure for roads and dry weather, the rolling resistance is lower and you are less prone to punctures. You might want to run the tyres a bit softer if you are going to be riding in muddy conditions.