Signatures

Is the signature on a PDF transmitted document deemed in ENGLISH law to be an original signature? I received such a Settlement Agreement from my counter-party which had to be executed within a time-limit. I signed it within the time limit and faxed it to a colleague who handed the fax to the counter-party but outside the time limit. The counter-party argues that for a contract to be "executed" the originals must be delivered and because they only gave us a PDF version they did not give an original signature. This seems nonsense to me but I would like chapter and verse and possibly legal case precedent to quote.