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It is still the case that condonation of a "matrimonial offence" can in some circumstances operate as a bar to a divorce petition. However, the courts are keen to encourage genuine attempts at reconciliation, and periods of cohabitation of up to six months can be disregarded in determining whether or not the marriage has irretrievably broken down.
Matrimonial Causes Act 1973 s.2(1)
One party to a marriage shall not be entitled to rely for the purposes of s.1(2)(a) above on adultery committed by the other if, after it became known to him that the other had committed that adultery, the parties have lived with each other for a period ... or periods exceeding six months.
Matrimonial Causes Act 1973 s.2(3)
Where ... the petitioner alleges that the respondent has behaved in such a way that the petitioner cannot reasonably be expected to live with him, , but the parties to the marriage have lived with each other for a period or periods after ... the final incident relied on ... that fact shall be disregarded ... if thelength of that period or of those periods together was six months or less.
Matrimonial Causes Act 1973 s.2(5)
In considering ... whether the period for which the respondent has deserted the petitioner or the period for which the parties to a marriage have lived apart has been continuous, no account shall be taken of any one period ... or periods (not exceeding six months in all) during which the parties resumed living with each other, but no period during which the parties lived with each other shall count as part of the period of desertion, or of the period for which the parties lived apart, as the case may be.
Bradley v Bradley  3 All ER 750, CA
W petitioned for divorce on the grounds of H's unreasonable behaviour. She complained of repeated violence towards her; the magistrates had made separation and maintenance orders which H had ignored. W continued to live in the matrimonial home with their six children, sleeping in H's bed and cooking his meals for fear of what he would do to her or the children if she refused. The judge dismissed W's petition on the basis that if W continued living with H, she could not logically claim that it was unreasonable to expect her to do so, but the Court of Appeal allowed W's appeal. She had remained with H because she had no option, not because it was a reasonable thing to expect: her petition should be allowed.
Matrimonial Causes Act 1973 s.3
(1) No petition for divorce shall be presented to the court before the expiration of the period of one year from the date of the marriage.
(2) Nothing in this section shall prohibit the presentation of a petition based on matters which occurred before the expiration of that period.
This is an absolute bar, though other forms of relief (such as a non-molestation order or an order for financial support) can be sought earlier.