Bump Update:
Without buying a brand new one (£100+), it looks exactly like this,
http://www.ebay.co.uk/itm/Genuine-BMW-E39-E38-Auxiliary-Radiator-Water-Pump-Electric-8381989-Tested-/321377865546?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item4ad39d0f4a
Would anyone know if the two are compatible? As the one for mine is posing a fruitless search.