Mike, did you notice that there was a Thomas Gilmore aged 22 with Robert in the 1871 census entry you mentioned?

Here is another marriage to look at - this time in Braddan parish register - it looks like a first marriage for your Robert, and again the details he gave would be helpful:
ROBERT SPROWSTON married JANE QUINE 28 FEB 1869 Braddan.
Jane SPRUSTON aged 18 buried Patrick 24 Oct 1869.

So these are my suggestions to find the further information you are looking for.
See:
Robert bapt 12 Dec.1847 Braddan son of Robert SPROOSTON & Mary GILMOUR. On LDS film no. 0106708.
Robert SPROWSTON married Jane QUINE 28 Feb.1869 Braddan. LDS film no.0106189.
Robert SPRUSTON married Jane LEECE 21 NOV 1871 Marown. LDS film no. 0106191

- Unless someone will look them up for you in the Manx Museum library.

Sue