Normally a transit Visa is not required through UK if you have a valid VISA for any of these countries:
US,Canada,Aus,NZ.
Coming for first time landing, we all satisfy this condition, since we have entry visa for Canada stamped on our passports.
However, if you remember, the immigration officer kind of invalidates our single entry Visa by striking it off with a pen.
Hence, for a return journey from Canada to India, I am not sure if this Visa is still considered valid.
A PR card is definitely accepted, but for a short landing and return, you would not have it.
I am more inclined to say that a transit Visa through UK is not required in this case well.
But I have my doubts as I am not totally sure about what will be the state of single entry Visa after it is struck off with a pen by immigration officer.
This situation can be considered very similar to this.
Let's say you were on work permit valid till 31st Dec 2016.
You board a flight in Toronto on 31st Dec evening.
But by the time you reach Heathrow, it is already afternoon on 1st Jan.
How is your Canadian work permit VISA treated then, for the purpose of deciding if you need Direct Air Side Transit Visa, valid or invalid?
This is the kind of grey area I am talking about.