First problem is that D/D's are initiated by the Company and not the Bank (unlike standing orders). and most Banks are unable to reinstate a D/D once it is cancelled. The only option would be to complete a new mandate which Sky would need to send you and return it back to them. They will then send this on to the Bank, which would give the Bank the authority to pay the D/D when Sky call for it. Most of the large companies are a member of the AUDDIS system, and some can set up D/D's their end, without the need to contact the Bank, and can do this over the telephone.
In conclusion..........Sky need to be taking the lead on this one, as the Bank cannot change any details, including reference numbers, on the D/D instruction.