Our 1 year old pump (replaced by British Gas - who wont come out FOC -
despite the pumping being fitted 1 year and 3 days ago) - is leaking
water from the electrical box on the side of the pump. This seems to
be in addition to the fact that the heating is not working (related?)

Just because it is over a year old, does not mean you are not entitles to a
replacement, or a repair.

Under the sale of goods act, things have to be
"fit for the purpose"
"of a satisfactory quality"
and "last for a reasonable amount of time"

You are not expected to replace a CH pump every year, so your pump has not
lasted for a reasonable time

I suggest you make a call to your local trading standards to see if you have
cause for complaint (I would think you do!)

you can find your local trading standards number here (along with other
information!)

www.tradingstandards.gov.uk

Sparks...