Without any great legal expertise, I would say a statutory instrument
is a legal power conferred by a statute. I'm not sure if statutory incident exists, but if it does, I think it would be an incident which would have to be reported under a certain statute, and I don't see how that could apply here. So I would stick to instrument.

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Note added at 23 mins (2010-03-28 11:33:01 GMT)
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In that case I would think the English text is wrong. I can find no example of "statutory incident" being used in any way that would make sense here, whereas "statutory instruments" would make sense.

This seems to be an older legal expression not much in use now, but used both in the US and the UK.

In the first document it hinges on statutes of ownership, but this is not the only useage as you will see in the marriage discussion.

"Appellant argues that the holding in Billings is "totally inapposite to the facts and law relevant here" because it involved the taxability of insurance proceeds of policies taken out before 1918, when the first provision expressly taxing life insurance proceeds was enacted, and the case was decided before 1942, when the statutory incidents of ownership test was enacted"

AND from the same document:

"The regulations, however, state that "incidents of ownership" has reference to the "rights of the insured . . . to the economic benefits" and then includes "the power to change the beneficiary" as one of several rights and powers the possession of which arguably constitutes a right to economic benefit."

There is more extended discussion of the legal meaning of incidents of ownership in this same document.

http://bulk.resource.org/courts.gov/c/F2/551/551.F2d.545.76-...

FROM the UK:

By contrast the 1980 Act preserved the contractual tenancy and merely added statutory incidents to it which overrode some of the contractual terms. Those overriding provisions included provisions which prevented it from being terminated except by a court order on the statutory grounds.

The rent Acts had always provided that when a tenant died, his security of tenure could be transmitted to his widow or family member. Since the Rent Act 1965 such a transmission could operate twice.

A person to whom a transmission was made became a statutory tenant. Under that system the provisions which limited the number of times the transmission mechanism could operate were extremely simple

http://business.timesonline.co.uk/tol/business/law/reports/a...

AND a more modern useage from the U.S.

"In its equal protection analysis, the court distinguished between the statutory incidents of marriage and the name "marriage." The court relied heavily on state sexual orientation discrimination and domestic partnership laws to decide that same-sex couples have a "strong interest" in treatment equal to that given to opposite sex couples. The court noted the difference between marriage and domestic partnership in terms of benefits provided and ease of entrance. Since the state had not offered any justifications for the marriage law other than "sustaining the traditional definition of marriage," the court found that there was no justification for the differentiation in benefits between married couples and domestic partners. The court rejected the state's proffered interest in preserving unity with the marriage laws of other states."

http://www.narth.com/docs/issued.html

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Note added at 2 hrs (2010-03-28 14:07:07 GMT)
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From 1966 Funk & Wagnalls Standard College dictionary:

incident
2. An event or action subordinate to some other.
3. Something that is characteristically or legally dependent upon or connected with another thing.