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Another question:

if i'd once been out of UK for 5 months at a go (student exchange), does this mean that i'd have to wait a further 5 months after 10 yrs since first entering the UK to start applying?

This sounds like another very grey area because although it doesn't seem to be stated on the UKBA website, that's what i heard from a visa advisor at the school i'm working in.

The rules state that you should not be outside the UK for a period of more than 6 months in one go, and that the total absences do not exceed 18 months in 10 years. 5 months outside the UK (particularly that you can demonstrate that you intended to come back) should not be a problem, as long as the total absences do not exceed 18 months.