10. Any covenant (not being a covenant affected by article 9) which would be enforceable by the Greater London Council immediately before the relevant date in respect of land within the extent of an area of housing accommodation transferred by article 3, being land which was sold or exchanged by the Greater London Council or by the London County Council and, immediately before such sale or exchange, was held by them for the purposes of the Housing Act 1957 or of any Act re-enacted by that Act, shall be of full force and effect in favour of the transferee authority.