On Mon, 18 Feb 2008 05:34:51 -0800 (PST), wrote:

Put new tap on but there was a leak from the old fitting under the
basin. I decided to replace the fitting, so i measured it and it seemed
like 13mm. I got replacement 15mm and 10mm fittings as there didn't seem
to be 13mm size, but these don't fit.

I'm not quite following what you mean by "fitting". Pictures are very
useful...

The house has various victorian features and still has lengths of lead
pipe in various places, including to the basin where it has the copper
pipe soldered in to go to the tap.

You may have imperial 1/2" pipe rather than modern metric 15mm but they
are normally close enough in outside diameter that it doesn't matter for
compression or sweated joints. 3/4" v 22mm is another matter though...

any ideas how to fix this or how to stop up the pipe?

Stop up the pipe with a suitably size push fit stop end. Check you get one
that can be released easily... You should be able to turn the water off at
the street stopcock on/near the boundary of your property.

--
Cheers
Dave.