The following may help those trying to trace illegitimate births on the IGI. It requires familiarity with the main IGI batch numbers for christenings in each parish (e.g. Braddan = C038021, Malew = C038091, Maughold = C038171, Arbory = C038181).
For years I had been trying to identify John Christian and Martha Christian, who appear in the Rushen 1851 census as grandchildren of Richard and Mary Christian in Kentraugh, aged 10 and 2.
Finally, I realised that by entering the first name "Martha" and the batch number for Rushen "C038251" on the IGI look up, one could get a list of Marthas from Rushen. I came up with 41, including Martha Wilson (christened March 11 1849, mother Jane Wilson, no father's name given).
Then, using Brian Lawson's 1851 census idata, I found that Jane Wilson was living at the same address as Richard, Mary, John and Martha. In 1861 there was also a Benjamin Christian aged 4 at the same address as Jane Wilson and Richard Christian
The same process worked for John Christian, although it was quicker when I narrowed the year range. He was christened John Kermode, son of Ann Kermode and John Christian and Ann and young John Kermode and Ann can be found in the 1841 census, at the same address in Rushen as Richard and Mary.
Jane Wilson appears in the 1881 census living with her son Benjamin Pulkinhorn (Cornish?).
By applying the above process, one finds Benjamin Wilson, christened in Rushen April 26 1856, son of Jane Wilson, no father's name given.
In the 1861 census he and Jane are living with Richard and Martha Christian (no Mary) in Rushen. He is named Benjamin Christian in 1861.
Hope this process helps
Bruce