This fall I want to go to India, and I need to stopover in London for
a couple of weeks on my way there. Given that is it likely to be
cheaper to try and find a flight that includes a stopover, or to
simply buy a cheap ticket to London, then buy my ticket to India from
there?

Given how cheap some round trips to London are, you'd probably do
better by getting a cheap ticket to London and buying a ticket to
India from there.

I have heard that you can get some great deals on plane tickets
in London, but haven't personally bought any, so have no idea where to
shop.
Anyway would appreciate any thoughts on this, or recommendations for
travel agents to try, either here on in the UK.

1. Try trailfinders.co.uk

Or in London:

2. Go to Earl's Court Road tube station, and on the street across from
that station there are lots of discount travel agencies.

3. Go to the neighborhood Southall, which has many immigrants from the
Indian subcontinent, and has travel agencies that specialize in
flights to India and Pakistan.