A method of providing one or more instances of a target double-stranded nucleic acid from a plurality of nucleic acid fragments, comprising: a plurality of initial hybridisation steps (H 1 -H 4), each initial hybridisation step comprising hybridising respective pairs of partially overlapping nucleic acid fragments to form a plurality of hybridised fragments; and one or more further hybridisation steps (H 5 -H 7), each further hybridisation step comprising hybridising respective pairs of partially overlapping hybridised fragments which are the direct product of a corresponding pair of earlier hybridisation steps to form longer hybridised fragments, where each of the pair of earlier hybridisation steps comprises one of the initial hybridisation steps or one of the further hybridisation steps; wherein said one or more further hybridisation steps comprise at least one further hybridisation step (H F) for which both of the corresponding pair of earlier hybridisation steps comprise an error-detecting type of hybridisation step (H E); the error-detecting type of hybridisation step (H E) comprising: performing an error detecting operation to detect whether the hybridised fragments formed in the error-detecting type of hybridisation step comprise at least one erroneous hybridised fragment comprising at least one mismatching base pair in an overlap region hybridised in the error-detecting type of hybridisation step; and discarding at least part of said at least one erroneous fragment to exclude the at least one erroneous fragment from a subsequent further hybridisation step; wherein the target double-stranded nucleic acid comprises a first strand of single-stranded nucleic acid hybridised to a second strand of single-stranded nucleic acid; and in each hybridisation step, the hybridised fragment of nucleic acid formed in that hybridisation step is bound to a surface of a reaction site via the first strand or the second strand. a plurality of initial hybridisation steps (H 1 -H 4), each initial hybridisation step comprising hybridising respective pairs of partially overlapping nucleic acid fragments to form a plurality of hybridised fragments; and one or more further hybridisation steps (H 5 -H 7), each further hybridisation step comprising hybridising respective pairs of partially overlapping hybridised fragments which are the direct product of a corresponding pair of earlier hybridisation steps to form longer hybridised fragments, where each of the pair of earlier hybridisation steps comprises one of the initial hybridisation steps or one of the further hybridisation steps; wherein said one or more further hybridisation steps comprise at least one further hybridisation step (H F) for which both of the corresponding pair of earlier hybridisation steps comprise an error-detecting type of hybridisation step (H E); the error-detecting type of hybridisation step (H E) comprising: performing an error detecting operation to detect whether the hybridised fragments formed in the error-detecting type of hybridisation step comprise at least one erroneous hybridised fragment comprising at least one mismatching base pair in an overlap region hybridised in the error-detecting type of hybridisation step; and discarding at least part of said at least one erroneous fragment to exclude the at least one erroneous fragment from a subsequent further hybridisation step; performing an error detecting operation to detect whether the hybridised fragments formed in the error-detecting type of hybridisation step comprise at least one erroneous hybridised fragment comprising at least one mismatching base pair in an overlap region hybridised in the error-detecting type of hybridisation step; and discarding at least part of said at least one erroneous fragment to exclude the at least one erroneous fragment from a subsequent further hybridisation step; wherein the target double-stranded nucleic acid comprises a first strand of single-stranded nucleic acid hybridised to a second strand of single-stranded nucleic acid; and in each hybridisation step, the hybridised fragment of nucleic acid formed in that hybridisation step is bound to a surface of a reaction site via the first strand or the second strand.