DWP contacted. Nice advisor said form had gone out on 9th January, same day as phone call but is sending out again. Extension given till 31st January. He has endorsed both DLA and PIP record not to suspend DLA payments whilst form(s) arrive/return. Stated that they use 2nd class post which is all over the place and can take up to 10 days to arrive. At least now feel happier that should original form comes within next few days I have time to deal with it. Thank you both for your advice.
