'Timewyrm: Exodus', written by Terrance Dicks, features the Seventh Doctor and Ace.
Back Cover Blurb
The pursuit of the Timewyrm leads the Doctor and Ace to London, 1951, and the Festival of Britain - a celebration of the achievements of this small country, this insignificant corner of the glorious Thousand Year Reich.
Someone - or something - has been interfering with the time lines, and in order to investigate, the Doctor travels further back in time to the very dawn of the Nazi evil. In the heart of the Germany of the Third Reich, he finds that this little band of thugs and misfits did not take over half the world unaided.
History must be restored to its proper course, and in his attempt to repair the timelines, the Doctor daces the most terrible dilemma he has ever known...