I would re-do it and fit the right size battens. Why bother going to all this trouble and not do a proper job. You get some rain blown up the tiles that leaks into the loft and you'll be cursing why the hell you didn't do it for ever and a day. It's a no brainer. This thread may be of interest //www.diynot.com/diy/threads/re-roof-getting-done-wrong-batten-size.420614/#3266472 The roofers may have better solutions but why can't you just get some diminishing sized battens made up for the last couple of feet to go from 25 to 18.
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