Slightly OT:How do I know what phase power supply my house has?

["clax01011971" <UKHA@xxxxxxxxxxxxxxxxxxxxxxxx>]

After reading about these on this list I was considering purchasing an
Electrisave (http://www.electrisave.co.uk) but
on the order page it
says "How many Additional Sensors would you like? (for 2 and 3 phase
supply)".

Do most houses only have a single phase and how do I tell what my
house has?

Thanks in advance,
Dave