As you are probably aware, the 40th went directly from Australia to Bombay in India in 1828, and remained there until September 1845. This means that it is highly likely that he married in India, assuming he didn't marry in Australia before 1828.
His service record says he spent 17 years 200 days abroad (Australia, India and Afghanistan etc) out of his total of 20 years and 146 days service, meaning he spent 21/2 years approximately in the UK. Up to one year of this may have been while he was getting his recruit training and waiting for a ship to go to Australia to join his Regiment. That still leaves at least another year and a half when he must have returned to the UK at some point, possibly during the first 10 years (1829 - 1839) in India since this was a quiet time for the Regiment. Unfortunately no manifests for the troopships which shuttled back and forth between India and the UK have survived so it may be difficult to confirm when he came back. I think it more likely that he came back at the end of his service especially since his discharge medical report says he was so unfit for further service due to rheumatism etc.
Again, as you are aware, if he married before 1837 you are relying on finding a parish register entry for any marriage. I assume that you have eliminated the half dozen or so Francis Northwood marriages in the UK between 1828 and 1846. Although there were parish registers compiled by the Army chaplains who conducted overseas marriages, I'm not sure what happened to them. They may be at the British Library as part of the East India Company records, but if they are, I would have expected them to have been indexed by FIBIS, but nothing turns up on the FIBIS database.
