I have to be honest after reading the above couple of posts I'm beginning to wonder whether or not there is anything to this after all or whether several different pieces of info have become conflated into a claim that there's going to be an announcement of lost Dr Who episodes.
What Mark Gatiss could be going to is a screening of these missing believed wiped episodes at the NFT tomorrow and one or more Pat troughton episodes that have been recovered are being shown (and by recovered I mean episodes we already know about), hence the 'Oh My Giddy Aunt'. The BBC DG meanwhile is giving his presentation and as was suggested above its got twisted.
To put it simply, nothing has been found and its just a screening in one place of a hitherto already rediscovered Pat Troughton episode whilst the BBC DG pontificates on Itunes and such like and if there is any announcement re missing Dr Who episodes it'll be on the lines of 'Sorry to disappoint but nothing has been recovered.' God I hope I'm wrong.