Season 22 ends on the 30th of March 1985, but as is well-documented Doctor Who is "rested" for 18 months, and the next season begins 70 weeks later on the 6th of September 1986. Once more the show has shifted back in the year and is now being broadcast during Autumn.
Season 23 has a number of firsts, not all welcome ones. Although it is not the first season to have an overall arc, it is the first (and last) to consist of one 14-week story - "Trial of a Time Lord". It is also at 70 weeks, the longest continuous period that Doctor Who has been off the air. Episode lengths have also moved back to 25 minutes, making this the shortest season of Doctor Who by individual episode length.
Various fan theories are put forward for the appallingly low audience figures for "Trial...": the 18-month hiatus chased viewers away; 14 weeks of continuity made it difficult for viewers to dip in mid-season; the 6th Doctor was still unpopular. Whatever the reasons, the fact is that Doctor Who was once more pitted against ITV's "A-Team" which after 2 years had firmly established itself with Saturday night audiences, whereas Doctor Who had been forcibly off the air for 18 months.
Quite simply audience figures start off very low in the first place, and more or less remain there - a good indication that viewers are going elsewhere from Day 1. The first week sees no more than 4.9 million viewers tuning in, and the most that can be said is that at least they're still there a week later.
By week 4 the audience has dropped to 3.7 million, making Part 4 of "Trial..." joint-second "least-watched episode of Doctor Who" along with Part 2 of Season 18's "Full Circle". Ironically it is only beaten by Part 3 of a story that has now gained cult status - Season 6's "War Games".
The season does climb back up to between 4 and 5 million viewers for the remainder of its 14 weeks, with the final story of "Trial..." (and unbeknown at the time, the 6th Doctor) peaking at 5.6 million. Unfortunately this still makes it the season with the worst average viewing figures (4.8 million) so far.