Hi Emma, and welcome to the BIMS forum.
I am surprised to read that the NHM does not have a 'resident expert'. They certainly do (several, in fact), but perhaps they no longer answer queries from the public.
My opinion, although likely to disappoint, is that this is not a meteorite. It does not look like any fresh fall that I have seen, and if it was a meteorite that had been buried in the soil for any length of time it would have had a distinctive surface layer of oxidised iron (effectively, rust). I agree that the surface does have a 'molten' appearance, which makes me think it might be a igneous rock, or even a man-made, industrial residue (slag-like). There obviously is iron in there, but many terrestrial rocks contain iron compounds and will attract a magnet. Finally, the interior of the rock looks very homogeneous, which I would not expect if it was a meteorite.
Hopefully other people with chip in with their opinions. In the meantime maybe you could have a browse through the following website to see if you can find anything similar?
http://meteorites.wustl.edu/meteorwrong ... wrongs.htmRegards, Kieron